The meeting "Dynamical Systems in Porto", to be held at the University of Porto from Setembro 04 to December 15, 2023, celebrates the diversity of themes studied by the different members of the Dynamical Systems group and aims the widespread of results, presentation of minicourses and intensification of scientific cooperation in the area. There will be some thematic activities, including both talks and/or minicourses.
A regular initiative of the UC|UP Graduate Program in Mathematics, the Summer School offers mini-courses by international experts working on topics related to the scientific scope of the Program.
PhD Defence
Structure, algorithmics and dynamics of endomorphisms for certain classes of groups
10:00 - room FC5 2.78, "Edifício dos Serviços Centrais", Faculty of Sciences - Univ. Porto (Rua do Campo Alegre 823)
April 28, 2023
PhD Defence
Ayk Telciyan - Analysis of equations of motion of inextensible strings and networks
14:30 - Sala José Anastácio da Cunha, DMUC
April 14, 2023
PhD Defence
Ana Belén Avilez García - A point-free study of z-embeddings, more general classes of localic maps, and uniform continuity
10:30 - Sala José Anastácio da Cunha, DMUC
April 14, 2023
PhD Defence
David João Brandligt de Jesus - Sharp regularity for degenerate fully nonlinear equations
15:00 - Sala 2.3, DMUC
April 13, 2023
Extremal Index and periodicity for chaotic dynamical systems
Ana Cristina Freitas (U. Porto)
16:00, Room 2.5, UC Math. Dept. (Seminar)
January 11, 2023
The extremal index appears as a parameter in Extreme Value Laws, characterising the clustering of extreme events. We apply this idea to a dynamical systems context to analyse the possible extreme value laws for the stochastic process generated by observations taken along dynamical orbits. We also present results about the limiting rare events points processes obtained in this context. Finally, we build dynamically generated stochastic processes for which the usual interpretation of the extremal index does not hold.
Tropicalizing moduli spaces
Margarida Melo (U. Roma Tre / CMUC)
14:00, Room 2.5, UC Math. Dept. (Seminar)
January 4, 2023
In algebraic geometry, the existence of moduli spaces to parametrize certain classes of objects is of central importance. Moreover, since these moduli spaces are often not compact, the construction of modular compactifications for theses spaces is very useful, as one can study them by using tools that are only available for proper spaces. In the last few years, it has been understood that often these compactifications depend on combinatorial data that can be given a tropical modular interpretation. When this is the case, one can study many properties of the original space by looking at its tropical counterpart.
In the talk, I will try to explain this interplay by looking at the guiding example of the moduli space of curves.
On the holonomy group
Hamza Bakhouch (PIUDM)
15:00, Room 2.5, UC Math. Dept. (Seminar)
January 4, 2023
In differential geometry, a differential structure on a topological space allows us to see locally our space like a Euclidean space. A foliation structure on a differential manifold allows us to see the manifold, locally as a family of "horizontal" (lines/planes/hyperplanes...) and then intuitively locally like a book, in which the dimension of the leaves(pages) is less than the dimension of the ambient manifold. The "dynamical" concept of Holonomy is very important to study the behavior of the leaves near a fixed compact leaf (if it exists). In this talk, I will briefly recall the notion of the fundamental group of a topological space, and a quick introduction of what is a foliated manifold. Furthermore, we will construct the holonomy group of a leaf and give some classical examples.
10h15-10h30 - Reception: André Carvalho and Lennart Obster
10h30-11h20 - Pedro Ribeiro: “A lemma connected with Special Functions having two indices - II”
11h30-12h20 - Gonçalo Varejão: "Towards a "dictionary" between commutative algebra and graph
theory"
12h30-14h00 Lunch Break
14h00-14h50 - Sebastián Alfonso: TBA
15h00-15h50 - Diogo Soares: "Computation of the commutation graph for the longest signed
permutation"
16h00-16h50 - Janet Flikkema (Radboud University) - Root systems and root data in the
classification of Lie algebras and algebraic groups
16h50- Coffee Break
PhD Defence
Herman Goulet-Ouellet - Schützenberger groups of minimal shift spaces
14:30 - Sala José Anastácio da Cunha, DMUC
December 14, 2022
Solving integro-differential problems with the Tau method: TauToolbox
Paulo Vasconcelos (U. Porto)
16:00, Room 2.5, UC Math. Dept. (Seminar)
December 14, 2022
The spectral Tau method delivers polynomial approximations to the solution of differential problems. The method tackles both initial and boundary value problems with ease and ensures excellent error properties, whenever the solution is smooth.
In this talk, the Lanczos’ Tau method is examined from a variety of aspects to provide a stable implementation for its operational version. The ultimate goal is to deploy a robust and efficient numerical library, the Tau Toolbox. This mathematical software enables a symbolic syntax to be applied to objects to manipulate and solve differential problems with ease and accuracy. Illustrative examples of the tool will be offered.
The congruent number problem
Nirina Albert Razafimandimby (PIUDM)
17:00, Room 2.5, UC Math. Dept. (Seminar)
December 14, 2022
In mathematics, especially in number theory, we often encounter problems that are easy to state, but whose solution is often extremely difficult and sometimes requires sophisticated methods from other branches of mathematics. The Congruent Number Problem is one such unsolved problem that goes back thousands of years. This problem asks whether a given natural number is the area of the right triangle with rational sides or not.
In my talk, I want to start by defining a congruent number, which allows us to state our problem, and I will discuss some of the current progress results. And then I'll move to introduce a notion of elliptic curve and its rational points, and I will discuss the relationship of this problem to the rational points on the elliptic curve.
Mathematical modeling and numerical simulation in stimuli responsive drug delivery systems
José Ferreira
16:00, Room 2.5, UC Math. Dept. (Seminar)
December 7, 2022
Stimuli responsive drug delivery systems are medical devices that are able to deliver drugs locally in the target tissue in a controlled way and without affecting the healthy tissues. The drugs are encapsulated in nanostructures that able to be safely transported to the target tissue where the drug release takes place enhanced by stimuli.
This talk aims to discuss the mathematical modeling, the numerical challenges and the numerical simulation of drug delivery systems triggered by exogenous stimuli.
Duality: from theory to applications
Célia Borlido (U. Coimbra)
16:00, Room 2.5, UC Math. Dept. (Seminar)
November 30, 2022
In 1936, Marshall H. Stone proved the so-called Stone's representation Theorem for Boolean algebras, thus establishing a duality between Boolean algebras and certain topological spaces. Inspired by Stone's result, many dualities between suitable categories of topological spaces and of partially ordered sets were later obtained. In this talk, I will start by informally explaining what a duality is and presenting some details of Priestley duality for bounded distributive lattices. We will then discuss some of its applications.
An introduction to the localic approach to topology
Jorge Picado (U. Coimbra)
16:00, Room 2.5, UC Math. Dept. (Seminar)
November 23, 2022
A topological space is a set of points along with a topology, a system of subsets called open sets that with the operations of union (as join) and intersection (as meet) forms a lattice with certain properties. In the localic approach to topology one forgets about points and thinks about a space as an abstraction of such lattices of open sets of points (more specifically, as a locale, that is, a complete lattice with an Heyting operator).
In this talk, we will show that the localic approach is naturally motivated and brings some new aspects to the development of topology. In particular, forgetting about points results in a working category with better properties and tools that make locales a less pathological counterpart of topological spaces.
Riemann surfaces, line bundles and divisors
Gabriel Martinho (PIUDM)
17:00, Room 2.5, UC Math. Dept. (Seminar)
November 23, 2022
Line bundles are fundamental objects in geometry, generalizing some properties of tangent and cotangent bundles of manifolds. In algebraic geometry, it is common to consider line bundles over compact Riemann surfaces. On the other hand, it is also frequent to make use of divisors on these surfaces, which, in general, are just integer-valued functions on a Riemann surface. They appear, for example, in the notable Riemann Roch theorem. In this talk, we explore the relation between divisors and line bundles over compact Riemann surfaces, while making use of the machinery provided by cocycles and Čech cohomology.
Diffusion models, involving differential equations with non-integer order derivatives, have lately become extremely popular because it is believed they can describe the observed reality more accurately than models based on integer order derivatives. They have been appearing in several fields, such as, physics, biology, engineering and finance. In this talk we start to present the definition of the fractional derivative and lead you to a physical interpretation of fractional diffusion.
Wave propagation in viscoelastic materials
Afonso Costa (PIUDM)
17:00, Room 2.5, UC Math. Dept. (Seminar)
November 16, 2022
Here we present a study of a problem that describes the displacement of a wave in a viscoelastic material. To the problem are established existence, uniqueness and stability results. Furthermore, a numerical method is proposed, a semi-discrete defined over a non-uniform partition and the fully discrete adding a temporal integration. It is proved that both methods have quadratic spatial convergence and, in the case of the discrete method, the convergence in time is of first order.
Hamilton's equations can be used to model many problems in the real world, and these equations could present several properties. Thus, in this talk, we are going to study numerical methods that preserve such properties, such as the Symplectic Euler Method.
The aim of my seminar is giving a brief introduction of basics in category theory and a quote to some applications of it. Precisely my seminar is divided into four parts. Some lines of the history of category theory, a quoting of some applications in several branches of mathematics, a short but detailed recap of the notions of category, functor between categories, natural transformation between functors, equivalence of categories and adjunction between functors. The definition of monad in a category and why this notion is important in mathematics is seen.
Modelling light propagation in the cornea
Milene Santos (PIUDM)
16:00, Room 2.5, UC Math. Dept. (Seminar)
November 2, 2022
To model the incidence and reflection of light in the cornea, we can use the Maxwell's equations. In this talk, we focus on the Maxwell's equations in the time-harmonic form which translates into the Helmholtz equation. We propose a numerical method based on nodal discontinuous Galerkin methods combined with a strategy that is specially designed to deal with curved domains which arise naturally in our domain of interest for the application.
The Riemann's Zeta function is a very interesting mathematical object, due to its relation to the prime numbers and to the Riemann's Hypothesis, proposed in 1859 and still to this day unanswered. In this seminar, we will see some equivalents to the famous hypothesis, involving the Möbius and Liouville functions.
3-manifolds, knots and surfaces
João Nogueira (CMUC)
16:00, Room 2.5, UC Math. Dept. (Seminar)
October 26, 2022
We will survey the motivation and history of 3-manifold topology, together with its twin subject of knot theory, giving emphasis to the importance of surfaces and groups on the understanding of these subjects. Under this framework, we will review some recent developments and guiding open questions.
The Karoubi envelope of a semigroup
Alfredo Costa (CMUC)
16:00, Room 2.5, UC Math. Dept. (Seminar)
October 19, 2022
The importance for Semigroup Theory of the Karoubi envelope of a semigroup became apparent in the 1980s with the Delay Theorem. This result fits in a framework that was since the 1960s one of the main driving forces of Semigroup Theory: the connections between semigroups and formal languages. In this talk we review some instances of this framework, highlighting a couple of applications of the Karoubi envelope of a semigroup.
PhD Defence
Igor Arrieta Torres - A study of localic subspaces, separation, and variants of normality and their duals
11:00 - Sala José Anastácio da Cunha, DMUC
PhD Defence
Carla Jesus - Spline-based numerical methods for fractional diffusion equations
14:30 - Sala José Anastácio da Cunha, DMUC
May 3, 2022
PhD Defence
Leonardo Larizza - On lax factorisation systems
15:00 - Sala José Anastácio da Cunha, DMUC
April 12, 2022
Simple Modules over skew polynomial rings
Paula Carvalho (CMUP, FCUP)
11: 30 (Room 004, UP)
January 13, 2022
We consider the structure of modules over Noetherian rings. A Noetherian ring S whose simple modules have the property that their finitely generated essential extensions are Artinian is said to satisfy property (?). For commutative Noetherian rings the validity of (?) is due to Matlis (1958).
In this talk we will discuss (?) for skew polynomial rings S = R[?; ?] where R is a commutative Noetherian ring and ? is an automorphism of R, with the indeterminate ? satisfying the relation ?r = ?(r)? for all r ? R.
A complete characterization is found when R is an affine algebra over a field K and ? is a K-automorphism. We will discuss in some detail the case when R = C[x, y] and ? is a C-algebra automorphism and indicate some open questions. This talk is based on a paper with Ken Brown and Jerzy Matczuk.
PhD Defence
Lili Song - Modeling Hessian-Vector Products in Nonlinear Optimisation: New Hessian Free Methods
15:00 - Sala José Anastácio da Cunha, DMUC
In the last decades non-Hermitian operators with real eigenvalues appeared in Quantum Mechanics (QM) and in other areas of Physics, e.g. Quantum Optics, Quantum Fluid Dynamics, and Quantum Field Theory, just to mention a few. We discuss the quantum mechanical setting of a system described by a Non-Hermitian Hamiltonian, pointing out the conflict raised by non-Hermiticity with the standard formalism of QM, and how to overcome it.
10h15-10h30 - Reception: André Carvalho and Beatriz Santos
10h30-11h20 - André Carvalho: “Endomorphisms of automatic groups”
11h30-12h20 - João Santos: "Symplectic Keys - Direct Way"
12h30-14h00 Lunch Break
14h00-14h50 - Pedro Silva: "Moduli spaces through the stack of triangles"
15h00-15h50 - Beatriz Santos: "The star-shaped and convex transform orders"
16h00-16h50 - Guest Speaker: Jan Philipp Wächter
16h50- Coffee Break
PhD Defense
Farrokh Razavinia - Quantum Generalized Heisenberg Algebras
15:30 - Room 030, Building FC1, UP Math. Dept.
November 26, 2021
An algebraic-topological approach to the classification of formal languages
Jorge Almeida (CMUP, FCUP)
11:30, Room 004, UP Math. Dept. (Seminar)
November 25, 2021
Representations of fundamental groups of surfaces
Peter Gothen (CMUP, FCUP)
11:30, Room 004, UP Math. Dept. (Seminar)
November 18, 2021
We consider representations of the fundamental group of a closed surface in the group SL(2,R). To each such representation one can associate an integer invariant called the Toledo invariant. A theorem of J. Milnor says that the absolute value of the Toledo invariant is less than or equal to g-1, where g is the genus of the surface, and a theorem of W. Goldman says that two representations can be continuously deformed into each other if and only if they have the same Toledo invariant.
In the seminar we shall explain the concepts and results mentioned above. Moreover, time permitting, we shal indicate how methods of holomorphic geometry can be used to study such questions through Higgs bundles.
A glance at Poisson manifolds, Courant algebroids and Lie-infinity algebras
Joana Nunes da Costa (CMUC, FCTUC)
14:30, Room 004, UP Math. Dept. (Seminar)
November 12, 2021
The goal of the talk is to give an overview of some aspects of Poisson geometry. I will start with the definition of Poisson manifold, then I will introduce Courant algebroids. Finally, I will present recent topics where I've been working on, related to Courant algebroids and Lie-infinity algebras.
3-manifolds, knots and surfaces
João Nogueira (CMUC, FCTUC)
11:30, Room 004, UP Math. Dept. (Seminar)
November 11, 2021
We will survey the motivation and history of 3-manifold topology, together with its twin subject of knot theory, giving emphasis to the importance of surfaces and groups on the understanding of these subjects. Under this framework, we will review some recent developments and guiding open questions.
Enriched functional limit theorems for chaotic dynamics and heavy tailed observables
Jorge Freitas (CMUP, FCUP)
12h00, Room 004, UP Math. Dept. (Seminar)
November 4, 2021
We consider stochastic processes arising from chaotic systems by evaluating an heavy tailed observable function along the orbits of the system. We prove the convergence of a normalised sum process to a Lévy process with excursions, designed to describe the oscillations observed during the clusters of extremal observations. The applications to specific systems include both hyperbolic and non-uniformly expanding systems.
Why I like noncommutative algebra: A tour of symmetry, geometry and mathematical physics
Samuel Lopes (CMUP, FCUP)
11h30, Room 004, UP Math. Dept. (Seminar)
October 28, 2021
I will start by motivating the study of noncommutative algebra and representation theory through examples rooted in symmetry, geometry and mathematical physics. Then I will talk about more specific aspects of my research interests.
From Krohn-Rhodes theory to Markov chains
Pedro Silva (CMUP, FCUP)
11h30, Room 004, UP Math. Dept. (Seminar)
October 21, 2021
In the early sixties, Krohn-Rhodes theory became a cornerstone of both finite semigroup theory and automata theory. The theory expanded subsequently in different directions, including the infinite setting and applications to the theory of Markov chains. The results mentioned are joint work with John Rhodes (Univ. of California at Berkeley) and Anne Schilling (Univ. of California at Davis).
Algebraic geometry with computer algebra
Carlos Rito (CMUP)
11h30, Room 122, UP Math. Dept. (Seminar)
October 14, 2021
The goal of the talk is to highlight the usefulness of modern computer algebra tools in the study of more classic algebraic geometry problems. I will give some examples related to my research on Schoen surfaces, Godeaux surfaces and the canonical map of surfaces of general type.
PhD Defense
Sara Joana Fino dos Santos Rodrigues de Carvalho
15:00 - Univ. Coimbra (by videoconference)
July 29, 2021
PhD Defense
António Pedro Neves Goucha
10:15 - Univ. Coimbra (by videoconference)
PhD Defense
Rúben Azevedo de Sousa
14:00 - Univ. Porto (by videoconference)
April 15, 2021
PhD Defense
Dieudonné Mbouna - On some Problems in the Theory of Orthogonal Polynomials
14:30 - Univ. Coimbra (by videoconference)
March 10, 2021
Research Seminar Program (RSP)
2020/21 second session
remote via: https://videoconf-colibri.zoom.us/j/83274945315
February 24, 2021
10h30-10h45 - Virtual Reception: Carla Jesus
10h45-11h30 - Rui Prezado: “Internal and enriched category theory”
11h30-11h45 - Break
11h45-12h30 - Igor Arrieta Torres: “Sublocale lattices and the T_D-duality”
12h30-14h00 - Lunch Break
14h15-15h00 - David Jesus: “Regularity Theory for Nonlinear Elliptic PDE”
15h00-15h45 - Carla Jesus: “The use of splines in fractional differential equations”
The full program of both sessions and some information on the speakers is available in
https://sites.google.com/view/rsp20202021/home.
Research Seminar Program (RSP)
2020/21 first session
remote via: https://videoconf-colibri.zoom.us/j/83274945315
February 17, 2021
10h30-10h45 - Virtual Reception: André Carvalho
10h45-11h30 - Leonardo Larizza: "Lax factorisation systems and categories of partial maps"
11h30-11h45 - Break
11h45-12h30 - Ana Belén Avilez García: "The frame of reals and a characterization of normal frames given by z-embedded sublocales"
12h30-14h00 Lunch Break
14h15-15h00 - Herman Goulet-Ouellet: "Tree sets and the Return Theorem"
15h00-15h45 - André Carvalho: "Endomorphisms of the product of two free groups"
15h45-16h00 - Break
16h00-16h45 - Special Guest: Tiago Cruz
The full program of the session and some information on the speakers is available in
https://sites.google.com/view/rsp20202021/home.
Why I like noncommutative algebra: A tour of symmetry, geometry and mathematical physics.
Samuel Lopes (CMUP, Univ. Porto)
11:15, Remote via Zoom https://videoconf-colibri.zoom.us/j/89645145648 (Seminar)
January 15, 2021
I will start by motivating the study or noncommutative algebra and representation theory through examples rooted in symmetry, geometry and mathematical physics. Then I will talk about more specific aspects of my research interests.
A new algorithm for the multiobjective minimum spanning tree
José Luís Santos (CMUC, Univ. Coimbra)
11:00, Room 2.4 , UC Math. Department (Seminar)
January 7, 2021
In this talk a new algorithm for the multiobjective minimum spanning tree problem is presented. It is based on a label algorithm for the multiobjective shortest path problem in a transformed network and can be used with any number of criteria. Some restrictions are added to the paths (minimal paths) in order to obtain a one-to-one correspondence between trees in the original networks and minimal paths in the transformed network. A short example is presented as well as some computational experiments were reported showing that the proposed algorithm outperforms the others algorithms existing in the literature. A deep study is also done about the number of nondominated solutions and a statistical model is presented to predict its variation with the number of nodes and criteria.
PhD Defense
Daniela Jordão - Coupling Hyperbolic and Parabolic IBVP: Applications to Drug Delivery
10:30 - Sala José Anastácio da Cunha, Univ. Coimbra
December 11, 2020
Commutation classes of the reduced words for the longest element of the symmetric group
Ricardo Mamede (CMUC, Univ. Coimbra)
11:00, Room 2.5 , UC Math. Department (Seminar)
December 10, 2020
Using the standard Coxeter presentation for the symmetric group, two reduced expressions for the same group element w are said to be commutationally equivalent if one expression can be obtained from the other one by applying a finite sequence of commutations. The commutation classes can be seen as the vertices of a graph G(w), where two classes are connected by an edge if elements of those classes differ by a long braid relation.
We will analyse properties of this graph for the longest element of the symmetric group, namely we compute the radius and diameter and show that it is not a planar graph for n>5. We also describe a family of commutation classes which contains all atoms, that is classes with one single element, and a subfamily of commutation classes whose elements are in bijection with standard Young tableaux of certain moonpolyomino shapes.
Laws of rare events for chaotic dynamics
Ana Cristina Moreira de Freitas (CMUP, Univ. Porto)
15:00, Remote via https://videoconf-colibri.zoom.us/j/89903116254 (Seminar)
December 2, 2020
The extremal index appears as a parameter in Extreme Value Laws, characterising the clustering of extreme events. We apply this idea to a dynamical systems context to analyse the possible extreme value laws for the stochastic process generated by observations taken along dynamical orbits. We also present results about the limiting rare events points processes obtained in this context.
The Schroedinger problem
Dmitry Vorotnikov (CMUC, Univ. Coimbra)
11:00, Room 2.4 , UC Math. Department (Seminar)
November 26, 2020
Schroedinger’s hot gas experiment aimed for determining the most likely evolution between two subsequent observations of a cloud of particles. Seemingly purely stochastic, this problem can be translated into a geometric language. Given a fixed functional on a Riemannian manifold, one can classically construct two canonical evolutionary processes: the associated gradient flow (a dissipative system) and Newton's equation (a Hamiltonian system). The Schroedinger problem can be viewed as a third "sibling" in this geometric family. We will discuss how to generalize this geometric problem to a more general metric space framework. Our outlook is inspired by the links between the Schroedinger problem and the Monge-Kantorovich optimal transport.
Statistical stability in chaotic systems
José Ferreira Alves (CMUP, Univ. Porto)
11:00, Remote via https://videoconf-colibri.zoom.us/j/83442305866 (Seminar) (CANCELLED)
November 19, 2020
We will make a brief introduction to the theory of dynamical systems, focusing our attention on the class of chaotic systems. We will pay special attention to the Lorenz attractor, well known for the "butterfly effect". We will see that, despite the impossibility of predicting the evolution of these systems from a deterministic point of view, we can have good information about their evolution from a statistical point of view, through physical probability measures.
Training reaction diffusion models for image restoration
Sílvia Barbeiro (CMUC, Univ. Coimbra)
11:00, Room 2.5 , UC Math. Department (Seminar)
November 12, 2020
Image restoration is one of the major concerns in image processing with many interesting applications. In the last decades there has been intensive research around the topic and hence new approaches are constantly emerging. Partial differential equation (PDE) based models, namely of non-linear diffusion type, are well-known and widely used for image noise removal. In this seminar we will start with a concise introduction about reaction diffusion models for image restoration. Then, we will discuss a flexible learning framework in order to optimize the parameters of the models improving the quality of the denoising process.
Unfolding a Bykov attractor: from an attracting torus to strange attractors
Alexandre Rodrigues (CMUP, University of Porto)
11:00, Room 2.5 , UC Math. Department (Seminar)
November 5, 2020
We present a comprehensive mechanism for the emergence of strange attractors in a two-parametric family of differential equations acting on a three-dimensional sphere. When both parameters are zero, its flow exhibits an attracting heteroclinic network (Bykov network) made by two 1-dimensional and one 2-dimensional separatrices between two hyperbolic saddles-foci with different Morse indices. After slightly increasing both parameters, while keeping the one-dimensional connections unaltered, we focus our attention in the case where the two-dimensional invariant manifolds of the equilibria do not intersect.
We prove the existence of many complicated dynamical objects, ranging from an attracting quasi-periodic torus, Newhouse sinks to Hénon-like strange attractors, as a consequence of the Torus Bifurcation Theory (developed by Afraimovich and Shilnikov). Under generic and checkable hypotheses, we conclude that any analytic unfolding of a Hopf-zero singularity (within an appropriate class) contains strange attractors.
Geometry and Topology of Higgs bundles moduli spaces
André Oliveira (CMUP)
11:00, Room 2.5 , UC Math. Department (Seminar)
October 29, 2020
A Higgs bundle over a Riemann surface is a pair consisting of a vector bundle together with some extra data. Their moduli spaces are quasi-projective varieties which naturally appear in many different areas of Mathematics and even of Physics. In this talk, we will take a general overview over two important examples where such moduli play a central role, namely on the study of components of spaces of representations of surface groups in a Lie group and also on the study of Mirror Symmetry.
An Introduction to Combinatorial Commutative Algebra
Jorge Neves (CMUC)
11:00, Room 2.5, UC Math. Department
October 22, 2020
Combinatorial Commutative Algebra comes of age following the works of Richard Stanley in the late seventies of the last century. A typical research problem consists of associating an ideal (or ring) to a given type of structure (simplicial complexes, graphs, polytopes, matroids, etc.) and to use this connection to study combinatorial properties via algebraic ones and vice-versa. We will give a concise introduction to this subject area, from its first developments through a recent research problem.
On orthogonal polynomials and their applications
José Carlos Petronilho (CMUC)
11:00, Room 2.5 , UC Math. Department (Seminar)
October 15, 2020
In this talk we will give an overview of the theory of orthogonal polynomials (OP), focusing on the so-called inverse problems. The problems to be considered arise in the framework of Sobolev OP and the theory of polynomial mappings. We will point out several applications, including connections with the so-called sieved OP (on the real line and on the unit circle), the spectral theory of Jacobi operators, and approximation theory, among others.
PhD Defense
Maria de Fátima Pina - Smoothing and Interpolation on the Essential Manifold
14:30 - Univ. Coimbra
June 29, 2020
PhD Defense
Mina Saee Bostanabad - A Semidefinite Approach to Algebraic Optimization
14:30 - Sala dos Capelos, Univ. Coimbra
February 17, 2020
PhD Defense
Jorge Valentim Soares - Extremal behaviour of chaotic dynamics
10:00 - Room 0.31, UP Math. Dept.
January 31, 2020
PhD Defense
Peter Jurriaan Lombaers - On some norm equations over cyclotomic fields
14:30 - Building FC1, UP Math. Dept.
January 13, 2020
Specht modules for general diagrams, their Schur functions, and Mondrian tableaux
Olga Azenhas (CMUC, Univ. Coimbra)
11:00, Room 0.04, UP Math. Dept. (Seminar)
December 18, 2019
The question of how to decompose a Specht module, for an arbitary diagram, into irreducible submodules, is not yet known. Similar question for Schur functions for arbitrary diagrams. We discuss known results for skew
shapes, in particular, border strips, or forests, and the role of R. Liu’s combinatorial interpretation of Mondrian tableaux, due to I. Coskun, in some of those constructions.
PhD Defense
David Mesquita - Entropy formulas for systems with singular sets
14:30 - Lecture Room FC1 005, UP Math. Dept.
11h15 - João Santos: Keys and Demazure crystals in type C
Abstract: We compute, mimicking the Lascoux-Schützenberger type A combinatorial procedure, left and right keys for a Kashiwara-Nakashima tableau in type C. These symplectic keys have a similar role as the keys for semistandard Young tableaux. More precisely, our symplectic key gives a tableau criterion for the Bruhat order of the hyperoctahedral group and describes the type C Demazure atoms and characters. The right and the left symplectic keys are related through the Lusztig involution.
12h00 - Diego Daltro: Mixing property in Dynamical Systems
Abstract: The purpose of this talk is to give an informal presentation of mixing property in the context of dynamical systems.
Lunch Break
14h15 - Rui Prezado: Representation theory of parabolic bundles
(joint talk with the CMUP Informal PhD Seminar)
Abstract: We explain how deformations of a parabolic bundle ξ are given by the vector space Ext1(ξ,ξ); moreover, for ξ stable, this is the tangent space at ξ for a certain moduli space of parabolic bundles. We show a possible way to approach these ideas from a representation theory point of view.
15h00 - Ali Moghanni: The rough interval shortest path problem
Abstract: Optimization problems have the potential to model a variety of real situations, in areas as diverse as transportation, network design or telecommunications. Very often vagueness, imprecise or uncertainty are encountered when defining such models, due to the fluctuation of the real problem parameters, like the traffic, the payload or even the weather. The rough set theory was introduced by Pawlak, by the end of the XX century, as a way to overcome this kind of issues and has raised increasing interest for its ability to represent complex or large data sets in a simplified manner.
In this talk we address the use of rough set theory when applied to network optimization. The approach is based on the use of rough intervals to characterize each parameter and the well-known shortest path problem is the considered problem. Basic concepts, like order relations and operations involved in this problem, are adjusted to a rough set paradigm and a labeling-like algorithm is presented to find a set of rough interval shortest paths between a pair of vertices in a network.
15h30 - Coffee break / Christmas meeting
ABOUT THE SPEAKERS:
João Santos is a PhD student of the Joint PhD Program UC|UP, working at the University of Coimbra, in Combinatorics, under the supervision of Professor Olga Azenhas.
Diego Daltro is a PhD student at Universidade Federal da Bahia (Brazil), working as a visiting student at the University of Porto, in Dynamical Systems, under the supervision of Professor Paulo Varandas.
Rui Prezado is a first year PhD student of the Joint PhD Program UC|UP.
Ali Moghanni is a PhD student of the Joint PhD Program UC|UP, working at the University of Coimbra, in Optimization, under the supervision of Professor Marta Pascoal.
Maths for crime prediction
Adérito Araújo (CMUC, Univ. Coimbra)
11:00, Room 0.04, UP Math. Dept. (Seminar)
December 11, 2019
It is undeniable that an unfortunate aspect of modern life, technologically and economically more developed, is the presence of crime distributed in major urban areas. This does not only affect countries like Portugal or the European continent, but is experienced in all countries around the world. Is possible to perceive that although crime has a ubiquitous character, this does not seem to be uniformly distributed both spatially and temporally.
In criminology, mathematical models can be very useful tools in the fight against crime. In fact, criminal activities have evolved in tandem with changes in technology. Crime has become more sophisticated, organised and transnational. With the changing nature of crime, the traditional approaches to tackle it are quickly becoming obsolete and there is a growing need for a new way of thinking to face that challenge head on.
In this seminar we consider a nonlinear cross-diffusion reaction model for the problem of prevention of residential burglaries and present some numerical results that show the importance of such models in real applications.
The moduli spaces of Higgs bundles
André Oliveira (CMUP)
09:30, Room 1.09, UP Math. Dept. (Seminar)
December 2, 2019
Given a Lie group G, the notion of G-Higgs bundle on a compact Riemann surface was introduced in the 1980s and 1990s by Nigel Hitchin. The algebraic varieties that parameterize these objects, i.e. their moduli spaces, have an extremely rich geometric and topological structure, yet are far from being understood for most groups G. Even their simplest topological invariant - the number of connected components - is still subject of current research. In this talk we will take a brief tour around Higgs bundles and their moduli spaces, avoiding going into technical details, and highlighting some of the exciting connections with other areas of Mathematics and Physics.
Emergence of strange attractors from singularities
Alexandre Rodrigues (CMUP, FCUP)
11:30, Room 004, UP Math. Dept. (Seminar)
November 27, 2019
In this seminar, we explore some mechanisms to obtain strange attractors (with one or two positive Lyapunov exponents). We will discuss some open questions related to the existence of strange attractors near a special type of homoclinic cycle (associated to a bifocus). This seminar is introductory and directed to non-specialists.
Corruption cycles in democracy
Alberto Adrego Pinto (FCUP)
11:00, Room 004, UP Math. Dept. (Seminar)
November 13, 2019
In this talk, with the goal of analysing the evolution of corruption, we introduce an evolutionary game theoretic dynamical model describing the interaction of citizens with government and officials or bureaucrats of the state in a democratic country.
We study and interpret the socio-political and economical characteristics of a society and their relationships with the evolution of corruption by means of the parameters of the model. These parameters represent the strength and weaknesses of the political institutions and of the agents of the game. We show the existence of a stable heteroclinic corruption cycle with increasing and decreasing periods of corruption by the government and officials. Citizen's voting power is the main mechanism provoking the decreasing periods in corruption. However, in a weak democratic state, the lack of political choices or vote buying might lead to a self-reinforcing mechanism of corruption where corruption is endemic and that may be seen as a social trap.
Dynamical systems from mutation-periodic quivers: definition and reduction by pre-symplectic tools
Inês Cruz (CMUP, FCUP)
11:00, Room 004, UP Math. Dept. (Seminar)
November 6, 2019
Cluster algebras and their associated quivers were introduced in 2002 by Fomin and Zelevinsky to provide a framework to study total positivity in matrix groups. Since then, cluster algebras have been successfully linked to a wide range of subjects including Poisson geometry, integrable systems, higher Teichmuller spaces, commutative and non commutative algebraic geometry.
The Laurent phenomenon exhibited by some particular recurrences (Gale-Robinson, Octahedron, Somos) has first been proved using this framework.
In this seminar I will explain how to obtain a recurrence from a mutation-periodic cluster algebra and present some results concerning reduction of the associated dynamical system to lower dimension by using pre-symplectic geometry.
This is joint work with Esmeralda Sousa-Dias (IST/CAMGSD).
Some challenging problems on numerical semigroups
Manuel Delgado (CMUP, Department of Mathematics, University of Porto)
11:00, Room 004, UP Math. Dept. (Seminar)
October 30, 2019
A numerical semigroup is just a co-finite submonoid of the nonnegative integers, under addition. Despite being so simple mathematical objects, they hide very challenging combinatorial and computational problems (among others, coming from various areas of mathematics). The aim of the seminar is to introduce the numerical semigroups (I plan to do it with the help of the GAP packages 'numericalsgps' and 'intpic'), and refer some of the challenges that captured my attention in the recent past.
Differential equations with two time-scales
Isabel Labouriau (CMUP, Department of Mathematics, University of Porto)
11:30, Room 004, UP Math. Dept. (Seminar)
October 30, 2019
I'll discuss the role of different time-scales in differential equations and the associated geometry. This will be illustrated by a simple example, the FitzHugh-Nagumo equations, and by two of these equations coupled to form a system in ${\mathbb R}^4$. When two different time-scales are considered separately, the dynamics is constrained by the geometry of the slow manifold, that in this specific example is a surface in the 4-dimensional phase space. Interesting dynamical behaviour arises at singularities of the projection of this surface into the plane of slow variables, shown in the picture. Even more interesting is the situation when the slow equation has a zero at a fold point.
Frankl Conjecture
André Carvalho (UC|UP PhD student)
11:00, Room 004, UP Math. Dept. (Seminar)
October 23, 2019
This seminar will deal with a conjecture proposed in 1979 by Peter Frankl related to union-closed families of sets. Despite the fact that the conjecture regards finite union-closed families of sets, which appear to be very simple objects, very little is known about them. We try to present in detail the main tools people have been using to approach the problem and unveil a little bit of the mystery behind these families.
Categorical semantics of intuitionistic linear logic
Carlos Fitas (UC|UP PhD student)
11:30, Room 004, UP Math. Dept. (Seminar)
October 23, 2019
The Curry-Howard-Lambek correspondence shows the deep connection between programs, proofs and morphisms in closed cartesian categories. It's natural to ask if similar results are true in other logics. Identifying the intuitionistic linear logic derivations up to the transformations in cut elimination, we obtain classes that spontaneously organize themselves into a monoidal category. Building on this we define a categorical semantics for intuitionistic linear logic and show that coherence spaces have the required categorical structure to model intuitionistic linear logic.
An ageing ordering
Beatriz Santos (UC|UP PhD student)
11:00, Room 004, UP Math. Dept. (Seminar)
October 16, 2019
Many classes of lifetime distributions are characterized by their ageing properties, that can be described by their survival functions or by their failure rate functions. These ageing properties allow us to compare two distributions within the same family, giving rise to ageing orderings. The aim of this work is to present an ageing ordering and some equivalent conditions and results that facilitate the comparison between two parallel systems with exponentially distributed components.
Ergodic theorems from topological and probabilistic viewpoints
Lucas Amorim (UC|UP PhD student)
11:30, Room 004, UP Math. Dept. (Seminar)
October 16, 2019
Ergodic theorems are classic measure theoretical results in dynamical systems or, more precisely, ergodic theory. They state that convergence of Birkhoff averages is typical, in a measure theoretical sense. This work aims to explain how these results can be reìnterpreted in light of topology and probability theory. The first relationship is presented through a Baire category analogue of a standard version of Birkhoff's ergodic theorem (assuming ergodicity). Instead of convergence of Birkhoff averages, the topological typical behavior will be the opposite: averages do not converge in a dramatic way. If time permits, we will also explore the second relationship, which examines how the law of large numbers interacts with Birkhoff's ergodic theorem (assuming ergodicity).
Deep learning meets PDEs: an effective framework for image restoration
Sílvia Barbeiro (CMUC, Department of Mathematics, University of Coimbra)
11:00, Room 004, UP Math. Dept.
October 9, 2019
Image restoration is one of the major concerns in image processing with many interesting applications. Partial differential equation (PDE) based models, namely of nonlinear diffusion type, are well-known and widely used for image noise removal. In this seminar we will describe a flexible learning framework based on the concept of optimized nonlinear reaction diffusion models for various image restoration problems.
Algebras associated to quivers
Rui Prezado (UC|UP PhD student)
11:00, Room 004, UP Math. Dept. (Seminar)
October 2, 2019
Given a small category I, its representation theory is, in essence, the study of the functor category [I,R-Mod] for some (not necessarily commutative) ring R. A case of interest is when we have I freely generated by a quiver Q. We construct an R-algebra RQ associated to this quiver Q in two distinct ways, then we sketch a proof that the aforementioned functor category is equivalent to the category of modules over RQ.
Spectral analysis in the study of dynamical systems
Raquel Couto (UC|UP PhD student)
11:30, Room 004, UP Math. Dept. (Seminar)
October 2, 2019
After a brief journey into a class of Symbolic Dynamical Systems, that is related to Markov Chains, I will present the Limit Theorem for Markov Chains which can be obtained as a corollary of Perron Theorem (a spectral theorem in linear algebra). Next, we will see what may be taken as a generalisation of these results in a more abstract setting, which, in particular, allows for the deduction of statistical properties of piecewise expanding maps of a closed interval.
Coupled cell networks and systems
Ana Paula Dias (CMUP)
11:00, Room 004, UP Math. Dept. (Seminar)
September 25, 2019
The aim of this talk is to introduce the connection between coupled cell networks and coupled cell systems. We also plan to address the issue of classifying networks from the dynamics point of view.
PhD Defense
Willian Ribeiro Valencia da Silva - Generalised enriched categories: exponentiation and injectivity
14:30 - Sala Anastácio da Cunha, DMat, Univ. Coimbra
This year the annual UC|UP PhD Summer School will take place in Braga and is jointly organised with the PhD Program in Applied Mathematics of the Universities of Aveiro, Minho and Porto (MAP-PDMA) and the PhD Program in Mathematics of the University of Aveiro (PDMat-UA).
The Summer School will consist of three intensive courses (8h each) and sessions where the students of the three PhD programs that organize the School may present their work.
Courses:
An introduction to Algebraic Geometry Francesco Polizzi (Università della Calabria, Italy)
Optimal control of sweeping processes Boris Mordukhovich (Wayne State Univ., Detroit, USA)
Morrey-Besov spaces, heat equations and Navier-Stokes equations Jan Vybíral (Czech Technical Univ., Prague,Czech Republic)
Scientific Committee:
Alexandre Almeida (Univ. Aveiro), Carlos Rito (Univ. Porto), Maria Irene Falcão (Univ. Minho), Susana Moura (Univ. Coimbra).
11h15 - Willian Ribeiro
Weak exponentiability in categories of lax algebras
Abstract: We start this talk by recalling, motivating, and formalizing in categorical terms the concept of exponentials. Then moving to the particular case of the category Top of topological spaces and continuous functions, we comment on the alternatives to the non-existence of arbitrary exponentials. Weak exponentials are one of these alternatives, and using the (T,V) setting introduced by Clementino and Tholen, these objects are proven to exist not only in Top, but also in many other categories from Analysis and Topology, as (probabilistic) metric spaces, bitopological spaces, approach spaces, and multi-ordered spaces, which are all topological over the category Set of sets and maps.
12h00 - Carla Dias
Gibbs-Markov-Young structures and stochastic stability
Abstract: In the 1960's, Sinai and Bowen showed that all smooth uniformly hyperbolic dynamical systems admit a finite Markov partition. Sinai, Ruelle and Bowen then used this remarkable geometric structure, and the associated symbolic coding of the system, to study ergodic properties such as the rate of decay of correlations. Some years ago, L.-S. Young proposed an alternative geometric structure, which we call Gibbs-Markov-Young (GMY) structure, as a way of studying the ergodic properties of certain dynamical systems. In this talk, we discuss the relation between GMY structure, Lyapunov exponents and stochastic stability in the setting of random perturbations. This is a joint work with J.F. Alves and H. Vilarinho.
Lunch Break
14h15 - Diogo Lobo
Image restoration models through backpropagation on medical imaging
Abstract: The use of neural networks for image processing tasks in the previous decade has set the pace of current research in this area. However, the general intrinsic "black box" nature of such algorithms is a drawback in the context of medical applications. We propose the use of deep learning techniques on already established non-linear diffusion schemes in order to optimize the parameters for each specific task. Consequently, we obtain improved image restoration models with good mathematical foundations. The learning framework and resulting models are presented along with related numerical results and image comparisons.
15h00 - Ahmed Elshafei
Geodesic completeness for Pseudo-Riemannian manifolds
Abstract: We study geodesic completeness of pseudo-Riemannian Lie groups by applying techniques from complex dynamics. We recall that, for a semi-simple Lie group, a geodesic corresponds to an integral curve in the Lie algebra of the Euler-Arnold vector field. These vector fields are algebraic and homogeneous of degree 2 thus amenable to be studied by techniques from complex dynamical systems. We present a complete study for SL(2,R) and give some indication on the further application of the techniques to SL(2,C).
ABOUT THE SPEAKERS:
Willian Ribeiro is a PhD student of the Joint PhD Program UCjUP, working at the University of Coimbra, in Category Theory , under the supervision of Professor Maria Manuel Clementino.
Carla Dias is a post-doc researcher of CMUP, working at the University of Porto, in Dynamical Systems.
Diogo Lobo is a PhD student of the Joint PhD Program UCjUP, working at the University of Coimbra, in Numerical Analysis, under the supervision of Professor Sílvia Barbeiro.
Ahmed Elshafei is a PhD student of the Joint PhD Program UCjUP, working at the University of Porto, in Geometry, under the supervision of Professor Helena Reis.
Truncated boolean representable simplicial complexes
Pedro V. Silva (UP)
14:30, Room 2.5, UC Math. Dept. (Seminar)
December 12, 2018
These structures constitute the largest known class of simplicial complexes admitting a geometric/topological theory. We will present the main features of this class and some recent results (joint work with Stuart Margolis (Bar-Ilan) and John Rhodes (Berkeley)).
Superalgebras with superinvolution
Herlisvaldo Costa Santos (student)
14:00, Room 2.5, UC Math. Dept. (Seminar)
December 12, 2018
Our objective in this work is to provide primitive associative superalgebras a structure analogous to those for the algebras and to classify primitive superalgebras with superinvolution having a minimal superideal.
Representation type of algebras and quivers
Ivan Yudin (CMUC)
14:30, Room 2.5, UC Math. Dept. (Seminar)
December 5, 2018
One of the first results one encounters in the standard university courses of algebra are classifications of equivalence classes of matrices under various transformations. If we are allowed to multiply the matrix in question from each side with an invertible matrix, then the corresponding equivalence class is fully determined by the rank of the matrix. If we consider equivalence classes of matrices under conjugations then two matrices are equivalent if
and only if their normal Jordan forms are the same up to permutation of blocks.
I will explain how these questions can be putted to a more general framework of Representation Theory of finite dimensional algebras and acyclic quivers. It turns out that above two examples fall into two out of three distinct typical situation:
- there are only finitely many building blocks for canonical form (in the case of simultaneous transformations of columns and rows these blocks are 1-times-1 matrices (0) and (1) );
- in each dimension there are finitely many one-parameter families of building blocks for canonical form (in the case of conjugation of matrices these blocks are precisely the standard Jordan blocks);
- the classification of equivalence classes is unfeasible.
At the end of the talk I will give a short report on a recent joint work with K. Erdmann and A.P. Santana, which deals with representation type of Borel-Schur algebras.
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11h00 - Maria Elisa Barbosa Silveira
Drug release enhanced by temperature: an accurate discrete model for solutions in H^3.
Abstract: In this talk we consider the coupling between two quasilinear diffusion equations: the diffusion coefficient of the first equation depends on its solution and the diffusion and convective coefficient of the second equation depend on the solution of the first one. This system can be used to describe the drug evolution in a target tissue when the drug transport is enhanced by heat. We study, from an analytical and a numerical viewpoints, the coupling of the heat equation with the drug diffusion equation. A fully discrete piecewise linear finite method is proposed for this system and we establish its stability. Assuming that the heat and the concentration are in H^3 we prove that the introduced method is second order convergent. Numerical experiments illustrating the theoretical results are also included.
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11h30 - Nuno Picado
Deciding about the emptiness of the interior of a manifold based on a sample of points
Abstract: In this talk, I will present some results about a method to decide, based on a sample of points from a manifold M, whether its interior is empty or not. This method was introduced in the article “Stochastic detection of some topological and geometric feature” by Cuevas et al. for independent random variables and sufficiently smooth manifolds. The decision process is based on an estimator constructed using balls centered at the points of the sample. We will show that the approach still holds if the sample satisfies a suitable dependence assumption. After this, the talk will be focused on the construction of models with the type of dependence needed for the results.
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12h00 - Dieudonné Mbouna
(M,N)-Coherent pairs of order (m,k) and Sobolev Orthogonal Polynomials on the non-uniform lattice
Maria Elisa B. Silveira is a PhD student of the Joint PhD Program UCjUP, working at the University of Coimbra, in Numerical Analysis, under the supervision of Professor José Augusto Ferreira.
Nuno Picado is a PhD student of the Joint PhD Program UCjUP, working at the University of Coimbra, in Probability and Statistics, under the supervision of Professor Paulo Eduardo Oliveira.
Dieudonné Mbouna is a PhD student of the Joint PhD Program UCjUP, working at the University of Coimbra, in Analysis, under the supervision of Professor José Carlos Petronilho.
A mathematical model for a plant circadian oscillator
Adérito Araújo (CMUC)
14:00, Room 2.5, UC Math. Dept. (Seminar)
November 28, 2018
Circadian rhythms are generated at the cellular level by a small but tightly regulated genetic network. They are observed in most organisms on earth and are known to play a major role in successful adaptation to the 24-h cycling environment. Understanding the molecular basis of cellular oscillations is more than an exercise in experimental genetics or biochemistry. Oscillators have characteristics (periodicity, robustness, entrainment) that transcend the properties of individual molecules or those interacting with each other through chemical reactions. These properties can only be fully understood by viewing experimental data from the perspective of quantitative mathematical modelling of chemical oscillatory processes. In the present talk we obtain a mathematical model to simulate the core loop of a plant - the Arabidopsis thaliana - circadian clock. Starting from the simple positive-negative feedback model, we aim to build a more comprehensive model that encompasses the key features of system.
Representations of fundamental groups of surfaces
Peter Gothen (CMUP)
15:30, Room 2.5, UC Math. Dept. (Seminar)
November 23, 2018
We consider representations of the fundamental group of a closed surface in the group SL(2,R). To each such representation one can associate an integer invariant called the Toledo invariant. A theorem of J. Milnor says that the absolute value of the Toledo invariant is less than or equal to g-1, where g is the genus of the surface, and a theorem of W. Goldman says that two representations can be continuously deformed into each other if and only if they have the same Toledo invariant.
In the seminar we shall explain the concepts and results mentioned above. Moreover, time permitting, we shall indicate how methods of holomorphic geometry can be used to study such questions through Higgs bundles.
On the existence of solutions for inextensible string equations
Ayk Telciyan (student)
12:30, Room 2.5, UC Math. Dept. (Seminar)
November 23, 2018
In talk thesis, we analyze existence of solutions for inextensible string equations. In particular, we have results in two directions.
On one hand, we find explicit traveling wave solutions for a system of hyperbolic conservation laws resulting from inextensible string equations via suitable change of variables. Then, we relate this solution with entropy and shock-wave solutions for which an established theory already exists.
On the other hand, we consider the problem with periodic boundary conditions and show local existence of solutions using well-studied results related to the wave equation.
In this seminar we will discuss some properties of a particular class of simplicial complexes from a purely combinatorial perspective. The elements of this class, including all the matroids, admit a representation by a matrix with entries in the boolean semiring, so they are said boolean representable (brsc). We will work with matrix and lattice representations of brsc, highlighting that, under mild assumptions, these two perspectives are equivalent. We will also introduce two notions of distance between suitable brsc and briefly explore them.
Campanato spaces and applications in partial differential equations
David Jesus (student)
11:30, Room 2.5, UC Math. Dept. (Seminar)
November 23, 2018
In this talk I will discuss a generalization of Campanato's characterization of Holder continuity, and use these results to study the regularity of solutions to the non-homogeneous p-Laplace equation.
Actions of Hopf algebras
Christian Lomp (CMUP)
11:30, Room 2.5, UC Math. Dept. (Seminar)
November 16, 2018
Group algebras and enveloping algebras of Lie algebras are examples of Hopf algebras. Automorphisms acting on rings as well as derivations acting on them are examples of Hopf algebra actions. Guided by these classical examples, we will motivate the study of Hopf algebra actions and consider in particular the question, when these actions are the only possible actions of a Hopf algebra on a certain class of rings.
A binary relation from a set A to a set B is a subset of the cartesian product AxB. Moreover, a function f: A->B can be identified with a particular type of relation, namely the relation consisting of pairs (x,f(x)). Consequently, the usual category of sets can be regarded as a subcategory of the category Rel(Set) whose objects are sets and morphisms are relations between them.
In this talk, we are going to present a categorical generalization of this situation due to P. Freyd and A. Scedrov, and explore its applications in category theory.
Not every category C is suitable for this: we will first characterize those categories in which we can still do relational calculus, that is, where we can still define its category of relations Rel(C).
More interestingly, we will see that ‘categories of relations’ (equivalently, their axiomatized version, ‘allegories’) provide an alternative and quite natural framework for understanding certain sorts of categories, e.g. regular categories, coherent categories, Heyting categories, and toposes. As a consequence, a nice corresponde will arise between ‘categorical structure’ and ‘allegorical structure’. Time permitting, we shall briefly discuss why a very large and important class of toposes can be built from fairly simple categories of relations.
Identification and simulation of flow in heterogeneous media: a model for oil extraction
Hugo Peña Gomez (student)
15:30, Room 2.5, UC Math. Dept. (Seminar)
November 14, 2018
The models of double porosity-permeability are used to described the dynamics of oil extraction (porous medium). The talk will be about presenting some aspects of this kind of models and a variation considering a particular medium structure. Some numerical results will be exposed.
Geodesics in the space of Kähler metrics
Pedro Silva (student)
14:30, Room 2.5, UC Math. Dept. (Seminar)
November 14, 2018
A Kähler manifold is a differentiable manifold with three geometric structures: a Riemannian metric, a symplectic form, and a complex structure, satisfying some compatibility conditions. The metrics and the symplectic forms for which these compatibility conditions are satisfied are called Kähler metrics and Kähler forms, respectively.
In this talk, we will show that, if we fix a complex structure on a compact Kähler manifold and if we choose a cohomology class for the Kähler forms, we can make the space of Kähler metrics into an infinite-dimensional manifold. Further, by giving this space the Mabuchi metric, we make it into a Riemannian manifold, for which we will study the geodesic equation. We will present an example of a geodesic curve in the case of the sphere. Time permitting, we will discuss how to obtain analytic solutions of the geodesic equation using an appropriate notion of complex flow.
On the group of a rational maximal bifix code
Alfredo Costa (CMUC)
15:00, Room 2.5, UC Math. Dept. (Seminar)
November 7, 2018
A code is a set of words that freely generates a free submonoid of the free monoid. A specially relevant place in the theory of codes is occupied by the maximal bifix codes.
In the last years, attention has been given to a process of "localization" in which one looks at the intersection of a code with a language of some special type.
To each rational code one associates in a natural way a special finite group that is one of the most relevant parameters in the study of rational bifix codes. In this talk, we present some results concerning the effects produced on this group by the aforementioned process of localization. We emphasize the methodology that we used, based on recent achievements concerning the structure of free profinite monoids.
Joint work with Jorge Almeida, Revekka Kyriakoglou and Dominique Perrin.
What is ... Pointfree Topology?
Jorge Picado (CMUC)
15:00, Room 2.5, UC Math. Dept. (Seminar)
October 31, 2018
Pointfree topology deals with frames and locales, the pointfree counterparts of topological spaces. We shall present motivation for this approach to topology and its basic features and facts. We will emphasize some of the main differences between the two approaches and some advantages of the pointfree approach.
3-manifolds, knots and surfaces
João Miguel Nogueira (CMUC)
14:30, Room 2.5, UC Math. Dept. (Seminar)
October 24, 2018
We will survey the motivation and history of 3-manifold topology, together with its twin subject of knot theory, giving special emphasis to the importance of surfaces on the understanding of these subjects. Under this framework, we will also review contributions of the speaker as suitable.
Applied Mathematics: applications in biomedicine and biomathematics
Isabel Narra de Figueiredo (CMUC)
14:30, Room 2.5, UC Math. Dept. (Seminar)
October 17, 2018
This talk focus on the description of some computer-assisted methods we have developed for the automatic interpretation of medical images and on some biomathematics models we have proposed to represent and simulate the early stages of colon cancer.
The boolean reflection of a frame and the Cantor-Bendixson derivative
Ana Belén Avilez (student)
15:00, Room 2.5, UC Math. Dept. (Seminar)
October 10, 2018
We consider two categories, the category of frames and the category of complete boolean algebras. We know that a complete boolean algebra is always a frame, but we want to assign universally a complete boolean algebra to each frame. It turns out this is not always true, but there is a result that tells us that a frame has a boolean reflection if and only if the tower of assemblies eventually stops. Remember that the assembly of a frame is the frame of all nuclei. What we really want to understand is how boolean a frame can be, and besides the boolean reflection, to understand this we can use the cantor-bendixson derivative which, in a way, shows us the boolean parts of a frame.
At the end, the purpose of this work is to understand the relation between frames and complete boolean algebras.
Computing the indecomposable representations of left regular bands
Herman Goulet-Ouellet (student)
14:30, Room 2.5, UC Math. Dept. (Seminar)
October 10, 2018
Left regular bands are a family of semigroups that enjoy many surprising properties. They have received some attention recently due to their link with Markov chains and hyperplane arrangements. In this talk, we will see how to compute the indecomposable projective representations of finite left regular bands. After introducing the basic definitions and tools needed, we will define left regular bands and give some of their classical properties. We will then give formulas (due to Franco Saliola, 2007) used to construct complete sets of primitive orthogonal idempotents in left regular bands algebras. These sets, called eulerian families, correspond to the indecomposable projective representations of left regular bands over arbitrary fields.
This year the annual UC|UP PhD Summer School will take place in Aveiro and is jointly organised with the PhD Program in Applied Mathematics of the Universities of Aveiro, Minho and Porto (MAP-PDMA) and the PhD Program in Mathematics of the University of Aveiro (PDMat-UA).
The Summer School will consist of four intensive courses, in the areas of Algebraic Combinatorics, Algebraic Number Theory, Numerical Analysis, and Mathematical Methods in Biology, and sessions where the students of the three PhD programs that organize the School may present their work. A precise schedule will be available soon.
Scientific Committee:
Alexandre Almeida (Univ. Aveiro), António José Machiavelo (Univ. Porto), Dirk Hofmann (Univ. Aveiro), José Augusto Ferreira (Univ. Coimbra).
PhD Defense
Anderson Feitoza Leitão Maia - Sharp regularity for the inhomogeneous porous medium equation
14:30 - Sala dos Capelos, Univ. Coimbra
March 21, 2018
Research Seminar Program (RSP)
2017/18 second session
Room 2.5, DMat UC
March 9, 2018
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11h30 - António Goucha
Phaseless rank and amoebas
Abstract: The phaseless rank of a nonnegative matrix M is defined to be the least k for which there exists a complex matrix N such that |N| = M, entrywise speaking. In optimization terms, it is the solution to the rank minimization of a matrix under phase uncertainty on the entries. This concept has a strong connection with some algebraic objects, called amoebas. An algebraic amoeba is the image of an algebraic variety under the absolute value map. In this talk we state some results related to phaseless rank and explore its connection with amoebas.
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Lunch Break
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13h30 - Rúben Sousa
Product formulas, generalized convolutions and integral transforms
Abstract: It is well-known that the ordinary convolution is closely related with the Fourier transform. It is therefore natural to ask: for other important integral transforms, can we define generalized convolution operators having analogous properties? Actually, the answer depends on the existence of a product formula for the kernel of the integral transform. In this talk, I will explain the general connection between product formulas, generalized convolutions and integral transforms. I will report on recent progress in constructing the product formula and convolution associated with the index Whittaker transform. Some applications will be presented, and the probabilistic motivation behind this work will be discussed.
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14h15 - Jorge Soares
A tour in extreme value laws
Abstract: In this talk, we analyse the stochastic process that arises from a dynamical system by evaluating an observable function along a given orbit of the system. Our goal is to give sufficient conditions for the existence of an Extreme Value Law for the considered process. We will present an example where the observable function is maximized in a Cantor Set and we will prove the existence of an Extreme Value Law with Extremal Index less than 1.
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António Goucha is a PhD student of the Joint PhD Program UCjUP, working at the University of Coimbra, in Optimization, under the supervision of Professor João Gouveia.
Rúben Sousa is a PhD student of the Joint PhD Program UCjUP, working at the University of Porto, in Analysis, under the supervision of Professor Semyon Yakubovich (Univ. Porto) and Professor Manuel Guerra (Univ. Lisboa).
Jorge Soares is a PhD student of the Joint PhD Program UCjUP, working at the University of Porto, in Dynamical Systems, under the supervision of Professor Jorge Milhazes de Freitas.
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PhD Defense
Muhammad Ali Khan - Statistical instability in chaotic dynamics
Room 031, Dep. Mathematics, Univ. Porto
March 9, 2018
PhD Defense
Khadijeh Alibabaei - On the profinite topology on groups and tameness of pseudo-varieties of groups
15h00m - Room FC1 029, Dep. Mathematics, Univ. Porto
March 1, 2018
PhD Defense
Fernando Lucatelli Nunes - Pseudomonads and Descent
10:00 - Sala dos Capelos, Univ. Coimbra
January 24, 2018
PhD Defense
Rui Sá Pereira - Forward-backward stochastic differential equations and applications
14h30m - Room 031, Dep. Mathematics, Univ. Porto
January 12, 2018
Jeux de tableaux and crystals
Olga Azenhas (CMUC)
15h30m, Room 108, UP Math. Dept. (Seminar)
December 12, 2017
My research is situated in the area of algebraic combinatorics. It focuses on developing combinatorial structures to encode complex objects as well as using combinatorial methods to break them into smaller pieces to manipulate them. One important feature of combinatorics is to reveal interesting connections between apparently unrelated objects. I will illustrate these ideas with some examples of my research interests.
Glimpses of noncommutative algebra and deformation theory, and some surprising connections to combinatorics
Samuel Lopes (CMUP)
15h30m, Room 108, UP Math. Dept. (Seminar)
December 5, 2017
I will discuss some of my research interests in relation to infinite-dimensional noncommutative algebras, highlighting the interplay with topics in ring theory, Hochschild cohomology and combinatorics.
Higgs bundles and the beauty of unexpected connections in Mathematics
André Oliveira (CMUP)
15h30m, Room 108, UP Math. Dept. (Seminar)
November 28, 2017
The theory of vector bundles over algebraic curves is a topic usually classified as a subarea of Algebraic Geometry, hence standing at the crossroads of several key areas of Mathematics and Modern Physics. D. Mumford constructed their moduli spaces more than 50 years ago, and since then much progress has been achieved in the geometrical description of these spaces. A new chapter has begun in the late 80's with the introduction of Higgs bundles over curves by N. Hitchin. On one hand, these moduli spaces have a very rich geometric and topological structure which is far from being fully understood. On the other hand, they play a crucial role in many different, apparently unrelated, areas, including representation theory, hyperkähler geometry, Langlands duality, mirror symmetry and more. In this talk I will briefly review some of these aspects, and present some results and open problems.
Nonlinear cross-diffusion models and applications
Adérito Araújo (CMUC)
15h30m, Room 108, UP Math. Dept. (Seminar)
November 21, 2017
After the pioneering work of Keller and Segel in the 1970s, cross-diffusion models became very popular in biology, chemistry, ecology, population dynamics, economy and physics to emulate systems with multiple species. Meanwhile, the underlying mathematical theory has been developed in a synergistic way with applications and, in recent years, this topic became the focus of an intensive research within the mathematics community. From a mathematical point of view, cross-diffusion models are described by time-dependent partial differential equations of diffusion or reaction-diffusion type, where the diffusive part involves a nonlinear non-diagonal diffusion matrix. This leads to a strongly coupled system where the evolution of each dependent variable depends on itself and on the others in a way governed by the diffusion matrix. Cross-diffusion terms are nowadays widely used in reaction-diffusion equations encountered in models from mathematical biology and in various engineering applications. In this talk we review the basic model equations of such systems and give an overview of their mathematical analysis with an emphasis on pattern formation. Finally we present numerical simulations in the context of relevant applications, namely, the dynamics of crime behaviour and medical image processing.
Nonlinear diffusion for image processing
Diogo de Castro Lobo (student)
15h30m, Room 108, UP Math. Dept. (Seminar)
November 14, 2017
The heat equation is probably the oldest and most investigated equation in image processing. We are going to discuss some approaches based on nonlinear diffusion for images denoising, namely the Perona-Malik and Complex Diffusion models. Numerical implementations and practical examples will be presented.
Evolutionary Game Dynamics
Ricard Trinchet (student)
16h15m, Room 108, UP Math. Dept. (Seminar)
November 14, 2017
Evolutionary game dynamics is the application of population dynamical methods to game theory and has been introduced by evolutionary biologists in the 1970s. In this talk, we will review some of the basic definitions and results of this field and motivate one of the possible dynamics which can be associated to a game: the replicator dynamics. Namely, we will see what is the relation between the Nash Equilibria and Evolutionarily Stable Strategies of a given game and the stable equilibria of its associated dynamics. We will try to illustrate all of these concepts with some examples.
Coupling ultrasound propagation and drug transport: a numerical approach
Daniela Jordão (student)
15h30m, Room 108, UP Math. Dept. (Seminar)
November 7, 2017
In this presentation we study a system of partial differential equations defined by a hyperbolic equation and a parabolic equation. The convective term of the parabolic equation depends on the solution and eventually on the gradient of the solution of the hyperbolic equation. This system arises in the mathematical modeling of several physical processes as for instance ultrasound enhanced drug delivery. In this case the propagation of the acoustic wave, which is described by a hyperbolic equation, induces an active drug transport that depends on the acoustic pressure. Consequently the drug diffusion process is governed by a hyperbolic and a convection-diffusion equation. Here, we propose a numerical method that allows us to compute second-order accurate approximations to the solution of the hyperbolic and the parabolic equation. The method can be seen as a fully discrete piecewise linear finite element method or as a finite difference method. The convergence rates for both approximations are unexpected. In fact we prove that the error for the approximation of the pressure and concentration is of second-order with respect to discrete versions of the H^1-norm and L^2-norm, respectively.
(student under supervision of Professor José Augusto Ferreira from UC)
Torsion Theories in Category Theory
Leonardo Larizza (student)
16h15m, Room 108, UP Math. Dept. (Seminar)
November 7, 2017
In the category of finitely generated Abelian Groups we have particular objects called torsion-free groups and torsion groups and any other objects can be decomposed as a direct sum of those particular groups. This behavior gives rise to an interesting classification for the objects of this category. Starting from this particular known example, I will present the categorical generalization of this situation. We will introduce Torsion Theories for Abelian Categories, outlining some basic properties, and I will briefly describe the case of Hereditary Torsion Theories. If time permits I will conclude describing the construction of Derived Torsion Theories for arrow categories.
Highlights in Dynamical Systems
Maria Carvalho (UPorto)
15h30m, Room 108, UP Math. Dept. (Seminar)
October 31, 2017
The aim of this talk is to present some relevant concepts and methods used in Dynamical Systems through a few interesting examples.
The spectral expansion approach to index transforms and connections with generalized convolutions and Lévy-type processes
Rúben Sousa (student)
15h30m, Room 108, UP Math. Dept. (Seminar)
October 24, 2017
Index transforms are integral transforms whose kernel depends on the parameters of well-known special functions. In this talk we will explain how these transformations can be systematically constructed via the classical spectral theory for Sturm-Liouville differential operators, and we will see how these spectral methods yield a general connection between the index transforms and the associated parabolic partial differential equations. Furthermore, we will show that this theory can be used to construct Lévy processes with respect to generalized convolution operators. Applications to pricing problems in mathematical finance will also be discussed.
(student under supervision of Professors Semyon Yakubovich from UP and Manuel Guerra from UL)
Drug release enhanced by temperature: mathematical perspective
Elisa Silveira (student)
16h15m, Room 108, UP Math. Dept. (Seminar)
October 24, 2017
The stimuli responsive polymers are long chain molecules that are sensitive to certain external stimuli - as temperature, chemical, light, electrical or magnetic field - and respond with observable or detectable changes in its properties.
In this talk, we study a quasilinear system of parabolic equations that can be used to simulate the drug transport enhanced by the temperature. In this case the first equation describes the temperature evolution and the drug transport is described by the second one. We observe that the diffusion coefficient of the second equation depends on the temperature.
We present fully discrete piecewise linear finite element methods that allow us to obtain numerical approximations for the temperature and concentration that present second order convergence with respect to a discrete L2 norm.
(student under supervision of Professors José Augusto Ferreira and Paula Oliveira, both from UC)
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11h00 - Peter Lombaers
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Integers and Ideals: There and Back Again
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Abstract: In number theory, when you try to solve an equation in a number field, it is often more convenient to work with ideals than with integers. This stems from the fact that ideals have unique factorization, but integers may not. I will explain the advantages and difficulties of this method using concrete examples.
Peter Lombaers is a PhD student of the Joint PhD Program UC—UP, working at the University of Porto, in Number Theory, under the supervision of Professor António José Machiavelo.
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13h30 - Willian Silva
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Introducing (T, V)-categories
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Abstract: In this seminar we introduce the concept of (T, V)-categories through its fundamental examples. In order to do so, we explore the concepts of monads and quantales, also with examples. We finish relating to the work on cartesian closed categories.
Willian Silva is a PhD student of the Joint PhD Program UC—UP, working at the University of Coimbra, in Category Theory, under the supervision of Professor Maria Manuel Clementino.
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14h30 -Mina Saee Bostanabad
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SOS versus SDSOS polynomial optimization
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Abstract: It is NP-hard to decide whether a polynomial is nonnegative, however, semidefinite programming can be used to decide whether a polynomial is a sum of squares of polynomials (SOS) in a practically efficient manner. In the context of polynomial optimization, it has become usual to substitute testing for nonnegativity with testing for SOS. Since there are much fewer sums of squares than nonnegative polynomials, we get only a relaxation and one that does not scale very well with the number of variables and degree of the polynomial. Recently, Ahmadi and Majumdar introduced a more scalable alternative to SOS optimization that they refer
to as scaled diagonally dominant sums of squares (SDSOS). The idea is searching for sums of squares of binomials, instead of general polynomials, which leads to a more scalable SOCP problem. In this presentation, we investigate the quantitative relationship between sums of squares of polynomials and scaled diagonally dominant polynomials. More specifically, we use techniques established by Blekherman to bound the ratio between the volume of the cones of these two classes of polynomials, showing that there are significantly less SDSOS polynomials than SOS polynomials. This drawback can be circumvented by using a recently introduced basis pursuit procedure of Ahmadi and Hall that iteratively changes the polynomial basis to a more suitable relaxation. We illustrate this by presenting a new application of this technique to an optimization problem.
Mina Saee Bostanabad is a PhD student of the Joint PhD Program UC—UP, working at the University of Coimbra, in Optimization, under the supervision of Professor João Eduardo Gouveia.
On Takens' Last Problem: times averages for heteroclinic attractors
Alexandre Rodrigues (U. Porto)
15h30m, Room 108, UP Math. Dept.
October 17, 2017
In this talk, after introducing some technical preliminaries about the topic, I will discuss some properties of a persistent family of smooth ordinary differential equations exhibiting tangencies for a dense subset of parameters.
We use this to find dense subsets of parameter values such that the set of solutions with historic behaviour contains an open set. This provides an partial affirmative answer to Taken's Last Problem (F. Takens (2008) Nonlinearity, 21(3) T33--T36). A limited solution with historic behaviour is one for which the time averages do not converge as time goes to infinity. Takens' problem asks for dynamical systems where historic behaviour occurs persistently for initial conditions in a set with positive Lebesgue measure.
The family appears in the unfolding of a degenerate differential equation whose flow has an asymptotically stable heteroclinic cycle involving two-dimensional connections of non-trivial periodic solutions. We show that the degenerate problem also has historic behaviour, since for an open set of initial conditions starting near the cycle, the time averages approach the boundary of a polygon whose vertices depend on the centres of gravity of the periodic solutions and their Floquet multipliers. In addition, further open questions will be discussed.
This is a joint work with I. Labouriau (University of Porto).
Reference: I.S. Labouriau, A. A. P. Rodrigues, On Takens' Last Problem: tangencies and time averages near heteroclinic networks, Nonlinearity 30(5), 1876-1910, 2017
ADI Methods for heat equation
Carla Jesus (student)
15h30m, Room 108, UP Math. Dept. (Seminar)
October 10, 2017
The production of metallic objects in three dimensions using selectivelaser melting (SLM) has become increasingly important in the manufacturing of small and complex molds. This technique uses digital information to produce a 3D metallic object through the action of a laser passing over a metal powder. In this talk, we will present a model that describes the heat transfer process during SLM. Due to not having access to the exact solution to most of these problems, we explore a numerical method to obtain an approximated solution to the problem. In particular, we will consider the alternating direction implicit (ADI) method that will be adjusted according to the different boundary conditions under consideration. We will present a convergence analysis of two bidimensional cases and conclude with an example of an ADI method in three dimensions.
Schur-Weyl duality: From the symmetric group to the general linear group
João Santos (student)
16h15m, Room 108, UP Math. Dept. (Seminar)
October 10, 2017
The Schur-Weyl duality is a way of relating irreducible representations of algebras, if these algebras are under some special conditions. In this talk, we will start with the definition of a representation with a goal to talk about these conditions and the relations given by the Schur-Weyl duality. In the end, we will see the most common example of this duality, relating the representations of two very well-known groups, the symmetric group and the general linear group.
In this talk, we will introduce a new and a weaker version of the famous Faddeev-Takhtajan-Volkov algebra in Sl_2 case and a complete calculation towards it by getting help of a new defined Poisson bracket just by using the Cartan matrix A_2, and then by employing this structure we will extend it to the Sl_3 case and so on to Sl_n case by using the Cartan matrix A_n.
Network dynamical systems - synchrony and graph fibrations
Oskar Weinberger (student)
16h15m, Room 108, UP Math. Dept. (Seminar)
October 3, 2017
Flows of vector fields (or any dynamical systems) associated to graphs/networks are often called network dynamical systems. These are systems that in some sense respect a graph structure, much like equivariant (symmetric) dynamical systems are systems that respect a group structure. Among various ways to formalise such systems, coupled cell systems is arguably the formalism which adopts the most algebraic and combinatorial perspective on the connection between networks and dynamics. In my talk I will try to motivate and give some basic definitions and results in this area. In particular, I want to present the notions of synchrony subspaces (invariant subspaces analogous to fixed point spaces for equivariant systems) and certain morphisms between graphs called graph fibrations. The main take away is that graph fibrations induce conjugacies on the level of vector fields. I will briefly sketch how this relates to ``hidden symmetries" of coupled cell systems, in the sense of the dynamics embedding as a synchrony subspace in the dynamics on another network with non-trivial self-fibrations. If time permits, I will also say a few words on some categorical aspects of the above.
PhD Defense
Pier Giorgio Basile - Descent theory of (T,V)-categories: global-descent and étale-descent
15:00 - Sala dos Capelos, Univ. Coimbra
This Short Course is part of the Course on PDEs, Dynamical Systems and Geometry of the Summer School of the UC|UP Joint PhD Program in Mathematics. It is organized by Professor José Miguel Urbano (jmurb@mat.uc.pt). It will be 10 hours long.
This Short Course is part of the Course on Statistics and Stochastic Processes of the Summer School of the UC|UP Joint PhD Program in Mathematics. It is organized by Professor Jorge Milhazes de Freitas (jmfreita@fc.up.pt). It will be 10 hours long.
This Short Course is part of the Course on Statistics and Stochastic Processes of the Summer School of the UC|UP Joint PhD Program in Mathematics. It is organized by Professor Carlos Tenreiro (tenreiro@mat.uc.pt). It will be 10 hours long.
This Short Course is part of the Course on PDEs, Dynamical Systems and Geometry of the Summer School of the UC|UP Joint PhD Program in Mathematics. It is organized by Professor Helena Reis (hreis@fep.up.pt). It will be 10 hours long.
PhD Defense
Deividi Ricardo Pansera - On semisimple Hopf actions
11h00m - Room 031, Dep. Mathematics, Univ. Porto
June 22, 2017
PhD Defense
Artur de Araújo - Representations of Generalized Quivers
Room 107, Dep. Mathematics, Univ. Porto
June 2, 2017
Research Seminar Program (RSP)
2016/17 second session
Room 2.4, DMat UC
June 1, 2017
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11h15 - Maryam Khaksar Ghalati
Two mathematical approaches in ophthalmology
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Lunch Break
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14h00 - Azizeh Nozad
Reducibility of nilpotent cone for G-Higgs bundle moduli space
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14h50 - Muhammad Ali Khan
Statistical instability for the contracting Lorenz flow
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Maryam Khaksar Ghalati is a former PhD student of the Joint PhD Program UC|UP, in the area of "Numerical Analysis and Optimization" under the supervision of professors Adérito Araújo and Sílvia Barbeiro.
Azizeh Nozad is a Postdoctoral fellow at Faculty of Science, University of Lisbon.
Muhammad Ali Khan is a PhD student of the Joint PhD Program UC|UP, working at the University of Porto, in the area of "Dynamical Systems" under the supervision of professor José Ferreira Alves.
Orthogonal polynomials, generalized coherent pairs of measures, and Sobolev orthogonal polynomials
Dieudonne Mbouna (student)
15h00m, UC Math. Dept. (Seminar)
May 31, 2017
In this talk we present results involving extensions of the concept of coherent pair of measures, introduced by A. Iserles, P. E. Koch, S. P. Norsett, and J. M. Sanz-Serna. In the first part of the talk we give a survey about basic results from the theory of orthogonal polynomials, (the inductive limit topology in the space of the polynomials, Favard’s theorem,the spectral theorem for orthogonal polynomials, Markov’s theorem, classical and semiclassical orthogonal polynomials, etc.). In the second part we present recent results involving the so-called (M,N)-coherent pairs of measures, and we give an application in the framework of Approximation Theory. We will use the above mentioned results to compute the Fourier-Sobolev coefficients appearing in the approximation of functions on appropriate Sobolev spaces.
The work for this presentation is concerned with the theory of estimation in manifolds. Here, we will not focus on the estimation of the set itself but on some of its topological properties. Based on a random sample of points from a manifold M, the goal is to achieve the exact description of topological properties of M, almost surely when the sample size increases to infinity. We will discuss two different models, depending on whether or not the observed points include noise.
An introduction to convexity properties and optimization methods for data analysis
Lili Song (student)
15h40m, UC Math. Dept. (Seminar)
May 24, 2017
First-order methods have been studied for convex optimization for many decades. They typically converge slowly, at rates that are only sublinear without strong convexity. However, the fact they demand little at each iteration have made them highly popular in the last decade for problems involving heavy data, such as those in information processing. Second-order methods are also becoming popular these days but for nonconvex problems such as those arising in deep learning.
In this talk we will cover how convexity is exploited in first-order methods, what are the main classes of algorithms and their
convergence rates. We will only cover the fully deterministic case where problems and algorithms are both deterministic.
An introduction to Clarke nonsmooth calculus and its application in optimization
Ali Moghanni Dehkhargani (student)
15h, UC Math. Dept. (Seminar)
May 17, 2017
In this seminar, we provide an overview of the main aspects of the Clarke nonsmooth calculus and cover a few of its applications to the type of optimization problems we have in mind.
The Clarke nonsmooth analysis has been widely applied due to the convenience of its generalized derivatives and to the convexity of its subdifferentials. We will cover the main concepts and results involved assuming that the functions are Lipschitz continuous.
Then we will briefly describe two applications to optimization algorithms, in one case by showing that Clarke generalized derivatives are nonnegative along limit directions of directional type methods.
In this seminar, our main purpose is to study the Mathematical Model of Glioma Growth and its Invasion. Cancer is a complex disease which leads to uncontrolled growth of abnormal cells, destruction of normal tissues and invasion of vital organs and gliomas are diffusive and highly invasive brain tumors. In our study, we will focus on the different aspects of cell tumor growth. We also describe the effect of chemotherapy on tumour growth. It is based on quantitative image analysis of histological sections of a human brain glioma and especially on cross-sectional area or volume measurements of serial CT images while the patient was undergoing chemotherapy. We also provide the Modelling Tumour Polyclonality and Cell Mutation.
Given an nxn nonnegative matrix M, its phaseless (signless) rank is the minimum rank of a complex (real) matrix whose entry-wise absolute value matrix is M.
We conjecture that, for any n, the boundary of nxn nonnegative matrices of phaseless rank less than n is contained in the union of coordinate planes and the set of nxn nonnegative matrices of signless rank less than n. In this session, we prove the n=3 case using two approaches: an analytical one and an algebraic one.
The spectral expansion approach to index transforms and connections with the theory of diffusion processes
Ruben Sousa (student)
15h40m, UC Math. Dept. (Seminar)
May 3, 2017
Index transforms are integral transforms whose kernel depends on the parameters ofwell-known special functions. In this talk we will explain how these transformations can be systematically constructed via the classical spectral theory for Sturm-Liouville differential operators, and we will see how these spectral methods yield a general connection between the index transforms, the associated parabolic partial differential equations and the corresponding diffusion processes. Applications of these results to pricing problems in mathematical finance will also be discussed.
Second order approximations for kinetic and potential energies in Maxwell's wave equations
Daniela Jordão (student)
17h10m, UC Math. Dept. (Seminar)
April 19, 2017
In this presentation we study a numerical scheme for wave type equations with damping and space variable coefficients. Relevant equations of this kind arise for instance in the context of Maxwell's equations, namely, the electric potential equation and the electric field equation. The main motivation to study such class of equations is the crucial role played by the electric potential or the electric field in enhanced drug delivery applications. The numerical method is based on piecewise linear finite element approximation and it can be regarded as a finite difference method based on non-uniform partitions of the spatial domain. We show that the proposed method leads to second order convergence, in time and space, for the kinetic and potential energies with respect to a discrete L^{2}-norm.
Geodesic flow on manifolds of non-positive curvature
Ahmed Elshafei (student)
16h30m, UC Math. Dept. (Seminar)
April 19, 2017
The study of geodesic flows on manifolds of non-positive curvature has been around for almost a century, it combines methods from geometry, dynamics, analysis and ergodic theory. This made the subject both more interesting and more difficult. In this seminar presentation the goal is to try to expose the subject using methods from Riemannian geometry and hyperbolic dynamics to show the interplay between these fields.
Various nonlinear evolutionary partial differential equations (coming from physics, fluid and solid mechanics, biology, chemistry etc.) can be viewed as geodesic equations or gradient flows on infinite-dimensional Riemannian structures.Understanding the underlying geometry of a PDE immediately provides certainconserved (in the case of geodesics) or dissipating (in the case of gradient flows) quantities which can be used, for instance, to get a priori bounds, to study asymptotic behaviour of solutions, or to develop numerical schemes. Further insights can be gained by observing that many of those Riemannian structures are related to the theory of optimal transport, which is enjoying a tremendous recent analytic progress. In my talk, I will try to introduce the students into this subject.
On the construction of complex algebraic surfaces
Carlos Rito (UP)
15h40m, UC Math. Dept. (Seminar)
March 8, 2017
Surfaces of general type are far from being classified. Frequently the construction of a single example is a challenge, even for the ones with low values of the invariants. The most efficient methods of construction are classical: quotients by the action of a group and coverings. In this talk I will review these and I will explain some of my recent constructions.
Extremal behavior of chaotic dynamical systems
Ana Cristina Moreira Freitas (UP)
15h, UC Math. Dept. (Seminar)
March 8, 2017
This talk is about the study of rare events for chaotic dynamical systems.We will address this issue by two approaches. One regards the existence of Extreme Value Laws (EVL) for stochastic processes obtained from dynamical systems, by evaluating an observable function (which achieves a global maximum at a single point of the phase space) along the orbits of the system. The other has to do with the phenomenon of recurrence to arbitrarily small sets, which is commonly known as Hitting Time Statistics (HTS). We will show the connection between the two approaches both in the absence and presence of clustering. Clustering means that the occurrence of rare events has a tendency to appear concentrated in time. The strength of the clustering is quantified by the Extremal Index (EI), which takes values between 0 and 1. The stronger the clustering, the closer the EI is to 0. No clustering means that the EI equals 1. Using the connection between EVL and HTS we associate the existence of an EI less than 1 to the occurrence of periodic phenomena.
PhD Defense
Maryam Khaksar Ghalati - Numerical Analysis and Simulation of Discontinuous Galerkin Methods for Time-Domain Maxwell’s Equations
Sala dos Capelos, Universidade de Coimbra
February 24, 2017
In this thesis we present a detailed analysis of a fully explicit leap-frog type discontinuous Galerkin (DG) method for the numerical discretization of the time-dependent Maxwell’s equations. The study comprehends models capable to deal with anisotropic materials and different types of boundary conditions. Despite the practical relevance of the anisotropic case, most of the numerical analysis present in the literature is restricted to isotropic materials. Motivated by a real application, in the present dissertation we consider a model which encompasses heterogeneous anisotropy, extending the existing theoretical results.
The DG formulation for the spatial discretization is developed in a general framework which unifies the study for different flux evaluation schemes. The leap-frog time integrator is applied to the semi-discrete DG formulation yielding to a fully explicit scheme. The main contribution of this thesis is to provide a rigorous proof of conditional stability and convergence of the scheme taking into account typical boundary conditions, either perfect electric, perfect magnetic or first order Silver- Müller absorbing boundary conditions and for different choices of numerical fluxes. The bounds
of the stability region point out not only the influence of the mesh size but also the dependence on the choice of the numerical flux and the degree of the polynomials used in the construction of the finite element space, making possible to balance accuracy and computational efficiency. Under the stability condition, we prove that the scheme is convergent being of arbitrary high-order in space and second order in time. When Silver-Müller boundary conditions are considered we observe only first order convergence in time. To overcome this order reduction we propose a predictor-corrector time integrator which is also analysed in this dissertation.
We illustrate the stability and convergence properties of the various schemes with numerical tests. The numerical results of our simulations support the theoretical analysis developed along the thesis.
On a regularity conjecture for degenerate elliptic pdes
José Miguel Urbano (UC)
15h, UC Math. Dept. (Seminar)
February 22, 2017
We establish a new oscillation estimate for solutions of nonlinear partial differential equations of degenerate elliptic type, which yields a precise control on the growth rate of solutions near their set of critical points. We then apply this new tool in the investigation of a longstanding conjecture which inquires whether solutions of the degenerate p-Poisson equation with a bounded source are locally of class C^{1,1/p-1}. This is a joint work with Eduardo Teixeira and Damião
Araújo.
Non Fickian diffusion in porous media
José Augusto Ferreira (UC)
15h40m, UC Math. Dept. (Seminar)
February 22, 2017
Transport processes in porous media have being described by the classical convection-diffusion equation for the concentration coupled
with an elliptic equation for the pressure and Darcy’s law for the velocity. Despite the popularity of this model, gaps between
experimental data and simulation results were observed in different scenarios. To overcome the limitation of the traditional diffusion
models, integro-differential models involving an integral in time were proposed. In this case, Fick’s law that defines a relation between the mass flux and the gradient of the concentration is replaced by an equation where the mass flux is given by the past in time of the
gradient of the concentration. In this talk, we present some numerical analysis results for IBVPs defined by integro-differential equations for the concentration and elliptic equations for the pressure.
The combinatorics of noncrossing and nonnesting partitions
Ricardo Mamede (UC)
15h, UC Math. Dept. (Seminar)
February 8, 2017
After a brief introduction to abstract Coxeter systems using a combinatorial approach based on words, we discuss recent developments on two combinatorial objects associated to the classical Coxeter groups: noncrossing and nonnesting partitions.
On the representation theory of some Noetherian algebras
Paula Carvalho (UP)
15h40m, UC Math. Dept. (Seminar)
February 8, 2017
I will present some classes of Noetherian algebras that I have been studying regarding their representation theory; down-up algebras and skew polynomial rings, presenting past and present results. If time allows, another class of algebras will be introduced for which it is unknown when they are Noetherian.
PhD Defense
Juliane Fonseca de Oliveira - Bifurcation of projected patterns
UP Math. Dept.
January 21, 2017
This thesis is related to the study of pattern formation in symmetric physical systems. The purpose of this thesis is to discuss a possible model, namely the projection model, to explain the appearance and evolution of regular patterns in symmetric systems of equations.
Results found in Crystallography and Equivariant Bifurcation Theory are used extensively in our work. In particular, we provide a formalism of how the model of projection can be used and interpreted to understand experiments of reaction-diffusion systems.
We construct a scenario where systems of symmetric PDEs posed in different dimensions can be compared as projection. In particular, we show how we can overcome the boundary conditions imposed by the problems.
We prove a correspondence between irreducible representations and fixed points subspaces, given by the action of a (n + 1)-dimensional crystallographic group, with the action of its projection on lower dimension. Such results are the first step to compare typical structures in dimension (n + 1), after projection, and the typical solutions of the posed problem in
dimension n.
We show that complex structures, as the black-eye pattern, obtained both as projection and as an experimental observation in CIMA reactions are the same. In particular, we believe that the projection model provides extra information to the study of pattern forming system, since it allows us to embed the original problem into one with more symmetry.
Castelnuovo-Mumford regularity and graph invariants
Jorge Neves (UC)
15h40m, UC Math. Dept. (Seminar)
December 16, 2016
The work of R. Stanley exposed a strong link between the theory of ideals of a polynomial ring generated by monomials and the theory of simplicial complexes. Recently, other bridges have been established between combinatorial structures and classes of ideals not necessarily generated by monomials. The Castelnuovo-Mumford regularity of a module over the polynomial ring is a basic invariant of the module. It is related with its free resolution and is, indeed, a measure of its complexity. It is natural to expect that, whenever a link between a class of ideals of a polynomial ring and a certain type of combinatorial structure exists, the Castelnuovo-Mumford regularity will translate into a meaningful combinatorial invariant. The focus of this talk will be on a recent joint work with A. Macchia (CMUC), Maria Vaz Pinto (CAMGSD, Lisbon) and Rafael Villarreal (Cinvestav, Mexico) in which the Castelnuovo-Mumford regularity of a particular class of ideals associated to graphs is equated with the combinatorics of the graph.
On Taken's Last Problem: times averages for heteroclinic attractors
Alexandre A. Rodrigues (UP)
15h, UC Math. Dept. (Seminar)
December 2, 2016
In this talk, after giving a technical overview about the topic, I will discuss some properties of a robust family of smooth ordinary differential equations exhibiting tangencies for a dense subset of parameters. We use this to find dense subsets of parameter values such that the set of solutions with historic behaviour contains an open set. This provides an affirmative answer to Taken's Last Problem (F. Takens (2008) Nonlinearity, 21(3) T33--T36). A limited solution with historic behaviour is one for which the time averages do not converge as time goes to infinity. Takens' problem asks for dynamical systems where historic behaviour occurs persistently for initial conditions in a set with positive Lebesgue measure. The family appears in the unfolding of a degenerate differential equation whose flow has an asymptotically stable heteroclinic cycle involving two-dimensional connections of non-trivial periodic solutions. We show that the degenerate problem also has historic behaviour, since for an open set of initial conditions starting near the cycle, the time averages approach the boundary of a polygon whose vertices depend on the centres of gravity of the periodic solutions and their Floquet multipliers. This is a joint work with I. Labouriau (University of Porto).
Singularities of vector fields and the dimension of groups of holomorphic diffeomorphisms
Helena Reis (UP)
15h40m, UC Math. Dept. (Seminar)
December 2, 2016
The group of holomorphic diffeomorphisms, Aut(M), of a compact complex manifold M is a Lie group of finite dimension. To provide bounds for the dimension of these groups is a classical problem in complex analysis. It is well known that the dimension of Aut(M) cannot be bounded in terms of the dimension of M solely. However several important problems arise once specific constraints are imposed on the manifold M. For example, the case of homogeneous manifolds has been intensively studied in connection with which is sometimes called Remmert conjecture. Another interesting situation corresponds to the case of algebraic manifolds whose Picard group is Z (the Hwang-Mok problem). There is an evident relation between bounds for the order of the zeros of holomorphic vector fields on M and bounds for the dimension of Aut(M). In this sense, results on singularities of holomorphic vector fields (particularly on specific questions concerning the extent to which a singularity of a vector field can be degenerate) have implications to problem mentioned above. We will discuss some concepts and recent results in this direction.
Research Seminar Program (RSP)
2016/17 first session
11h, Room M005, UP Math. Dept. (Research Seminar Program)
November 25, 2016
MORNING SESSION - Algebra, Combinatorics and Number Theory
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11:00h Antonio Macchia
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Title Proper divisibility as a partially ordered set (*)
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Abstract We define the order relation given by the proper divisibility of monomials, inspired by the definition of the Buchberger graph of a monomial ideal. From this order relation we obtain a new class of posets. Surprisingly, the order complexes of these posets are homologically non-trivial. We prove that these posets are dual CL-shellable, we completely describe their homology (with integer coefficients) and we compute their Euler characteristic. Moreover this order relation gives the first example of a dual CL-shellable poset that is not CL-shellable.
(*) joint work with Davide Bolognini, Emanuele Ventura and Volkmar Welker
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12:00h Alberto José Hernandez Alvarado
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Title The Quotient Module, Coring Depth and Factorisation Algebras
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Abstract In this conference, I will be reviewing the main aspects of my thesis dissertation. I will introduce the notion of depth of a ring extension $B\subseteqA$ and give several examples as well as important results of recent years. I will then consider a finite dimensional Hopf algebra extension $R \subseteq H$ and its quotient module $Q := H/R^+H$ and show that the depth of such an extension is intrinsically connected to the representation ring of $H, A(H)$. In particular, we will see that finite depth of the extension is equivalent to the quotient module $Q$ being algebraic in $A(H)$. Next, I will introduce entwining structures and use them to show that a certain extension of crossed product algebras is a Galois coring and use that to give a theoretical explanation for a result of S. Danz (2011). Finally, I will discuss factorisation algebras and their roll in depth, in particular a result on the depth of a Hopf algebra $H$ in its generalised factorised smash product with $Q^{*op}$.
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***LUNCH BREAK***
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AFTERNOON SESSION - Algebra, Logic and Topology
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14:30h Pier Giorgio Basile
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Title A lax version of the Eilenberg-Moore adjunction (*)
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Abstract In Category Theory there is a well developed theory of monads, proved to be very useful for 1-dimensional universal algebra and beyond. The relation between adjunctions and monads was first noticed by Huber (Homotopy Theory in General Categories): every adjunction gives rise to a monad. Then, Eilenberg, Moore and Kleisli realized that every monad comes from an adjunction. In particular, Eilenberg and Moore (Adjoint Functors and Triples) realized that, for every monad T, there is a terminal adjunction (called Eilenberg-Moore adjunction) which gives rise to T. Category Theory can be also developed in a 2-dimensional case, that is, considering not only morphisms between objects but also morphisms (usually called 2-cells) between morphisms themselves. Thereby, one can study lax versions of the theory of monads. In the pseudo version, that is when we replace commutative diagrams by coherent invertible 2-cells, the relation between biadjunctions and pseudomonads has been investigated by F. Lucatelli Nunes in the paper On Biadjoint Triangles as a consequence of the coherent approach to pseudomonads of S. Lack. The next step consists of studying the lax notion of monads, in which the associativity and identity works only up to coherent (not necessarily invertible) 2-cells. In this talk we present a work in progress where we try to generalize to the lax-context the classical result of Eilenberg-Moore. For this purpose, having in mind the notion of lax extension of monads introduced and studied in the context of Monoidal Topology (Metric, topology and multicategory: a common approach - M.M. Clementino and W. Tholen), we use a generalization of Gray's lax-adjunction (see the monograph Formal Category Theory). Then, we show some steps of the construction leading to the positive answer.
(*) joint work with Fernando Lucatelli Nunes
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15:30h Fernando Lucatelli Nunes
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Title Kan construction of adjunctions
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Abstract I will talk about a basic procedure of constructing adjunctions, sometimes called Kan construction/adjunction. In the first part of the talk, I will construct abstractly such adjunctions via colimits. In the second part, we give some elementary examples: fundamental groupoid, sheaves, etc. We assume elementary knowledge of basic category theory (definition of categories, colimits and Yoneda embedding).
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ABOUT THE SPEAKERS
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1. Antonio Macchia is a PosDoc researcher working at the University of Coimbra in the area of "Algebra and Combinatorics".
2. Alberto José Hernandez Alvarado is a former PhD student of the Joint PhD Program UC|UP working at the University of Porto in the area of "Algebra, Combinatorics and Number Theory" under the supervision of professor Lars Kadison.
3. Pier Giorgio Basile is a PhD student of the Joint PhD Program UC|UP working at the University of Coimbra in the area of "Algebra, Logic and Topology" under the supervision of professor Maria Manuel Clementino.
4. Fernando Lucatelli Nunes is a PhD student of the Joint PhD Program UC|UP working at the University of Coimbra in the area of "Algebra, Logic and Topology" under the supervision of professor Maria Manuel Clementino.
A meeting between Sobolev, Bessel, Hölder, Lorentz and Karamata in order to discuss optimal embeddings and some open problems
Júlio Neves (UC)
15h, UC Math. Dept. (Seminar)
November 18, 2016
In this talk we give a short survey on the results of embeddings of Sobolev type spaces into Hölder type spaces, including as well the famous result of Brézis and Wainger about almost Lipschitz continuity of elements of the Sobolev space with super-critical exponent of smoothness. Afterwards, we discuss different sharp inequalities that will allow to derive necessary and sufficient conditions for embeddings of Sobolev type spaces modelled upon rearrangement invariant Banach function spaces X into generalized Hölder spaces. Some open problems will be presented as well.
Smoothness Morrey spaces and their envelopes
Susana Moura (UC)
15h40m, UC Math. Dept. (Seminar)
November 18, 2016
The classical Morrey space {\cal M}_{u,p}(R^n), 0 < p \leq u < \infty, is defined to be the set of all locally p-integrable functions f such that \|f \mid {\cal M}_{u,p}(R^n)\| :=\, \sup_{x\in R^n, R>0} R^{\frac{n}{u}-\frac{n}{p}} \left(\int_{B(x,R)} |f(y)|^p \;\mathrm{d} y \right)^{\frac{1}{p}} is finite, where B(x,R) denotes the ball centered at x\in R^n with radius R>0. They are part of the wider class of Morrey-Campanato spaces and can be considered as an extension of the scale of L_p spaces. Built upon these basic spaces Besov-Morrey spaces \cal N^{s}_{u,p,q} and Triebel-Lizorkin-Morrey spaces \cal E^s_{u,p,q} attracted some attention in the last years, in particular in connection with Navier-Stokes equations. Closely related to theses scales are the spaces of Besov type B_{p,q}^{s,\tau} and Triebel-Lizorkin type F_{p,q}^{s,\tau}, \tau\geq 0, which coincide with their classical counterparts when \tau=0. We present a survey on such different scales of smoothness spaces of Morrey type. We also introduce the general concept of growth and continuity envelope of a function space and determine the envelopes of the above mentioned spaces. In some cases a specific behaviour appears which is different from the "classical" situation in Besov or Triebel-Lizorkin spaces. This talk is based on joint work with D.D. Haroske (Jena), L. Skrzypczak (Poznan), D. Yang (Beijing) and W. Yuan (Beijing).
Ring theoretical and combinatorial aspects of representation theory
Samuel Lopes (UP)
15h, UC Math. Dept. (Seminar)
November 4, 2016
I will discuss some of my research interests in relation to representation theory of infinite-dimensional algebras, highlighting the interplay with topics in ring theory and combinatorics.
A successful pair: symbolic dynamics and (pro)finite semigroups
Alfredo Costa (UC)
15h40m, UC Math. Dept. (Seminar)
November 4, 2016
My research has been mostly about exploring natural links between the field of symbolic dynamics and the theory of finite and profinite semigroups, with applications in both directions. The aim of this talk is to give examples of relevant results and open problems stemming from this line of research.
Diverse paths in network optimisation
Marta Pascoal (UC)
15h, UC Math. Dept. (Seminar)
October 28, 2016
A path in a graph is a sequence of vertices linked by graph edges. The determination of an optimal path with respect to a given objective function has led to classical network optimization problems. However, for many applications, it is useful to know an alternative backup solution,which can replace the optimal path if needed and, in some cases, which is reasonably diverse (or "different") from the latter. In this talk we will discuss applications which justify finding diverse
paths, approaches that have been used to model and solve such problems, as well as open questions within this line of research.
Positive semidefinite rank and representations of polytopes
João Gouveia (UC)
15h40m, UC Math. Dept. (Seminar)
October 28, 2016
Let M be a p-by-q matrix with nonnegative entries. The positive semidefinite rank (psd rank) of M is the smallest integer k for which there exist positive semidefinite matrices A_i,B_j of size k×k such that M_{ij}=trace(A_i,B_j). The psd rank was recently introduced, and has has many appealing interpretations, capturing some geometrical aspects of the power and limitations of semidefinite programming. In this talk, we will briefly cover basic properties and open questions on this quantity, and proceed to use some of these results to provide a complete characterization of polytopes in R^4 that can be represented as economically as possible by means of a semidefinite program.
A fractional diffusion model with resetting
Ercília Sousa (UC)
15h, UC Math. Dept. (Seminar)
October 14, 2016
We consider a fractional partial differential equation that describes the diffusive motion of a particle, performing a random walk with Lévy distributed jump lengths, on one dimension with an initial position x0. The particle is additionally subject to a resetting dynamics, whereby its diffusive motion is interrupted at random times and is reset to x0. A numerical method is presented for this diffusive problem with resetting. The influence of resetting on the solutions is analysed and physical quantities such as pseudo second order moments will be discussed. Some comments about what happens in the presence of boundaries will be also included. This talk is based on joint work with Amal K. Das from Dalhousie University (Canada).
When assigned with the task of image reconstruction, the first challenge one faces is the derivation of a truthful model for both the information we want to extract and the data. The natural question arises: how can we make our model adaptive to the given data? Diffusion processes are commonly used in image processing in order to remove noise. The main idea is that if one pixel is affected by noise, than the noise should be diffused among the neighboring pixels in order to smooth the region. In this way, proper diffusion partial differential equations (PDE) have been considered to achieve this end. The choice of the diffusion parameter plays a very important role for the purpose of denoising. Roughly speaking, one wants to allow diffusion on homogeneous areas affected only by noise and to forbid diffusion on edges to preserve features of the original denoised image. Consequently, the efficient models exhibit solution-dependent adaptivities in form of nonlinearities or non-smooth terms in the PDE. After a critical discussion of models based on nonlinear diffusion, we will turn towards the second modelling strategybased on nonlinear complex diffusion, which is suggested taking into account its advantages with regard to edge preservation, speckle filtering capabilities and potential to recover the original (uncorrupted) signal. The models will be compared by means of illustrative practical examples. Some applications of the complex diffusion filter, namely for despeckling optical coherence tomograms from the human retina, will be highlighted.
PhD Defense
Alberto Hernandez - The Quotient Module, Coring Depth and Factorisation Algebras
Room 107, Dep. Mathematics, Univ. Porto
September 30, 2016
In Boltje-Danz-Kulshammer, J. of Algebra, 03-019, (2011) it was shown that for a finite group algebra extension over any commutative ring the depth is always finite. Later, in Kadison, J. Pure and App Algebra, 218: 367-380, (2014) depth of such a subgroup pair was obtained by computing on the permutation module of the left or right cosets. This holds more generally for finite dimensional Hopf algebra extensions. We show that the depth of the Hopf subalgebra pair R H is related to the depth of its generalised permutation module Q := H=R+H in its module category. Furthermore we establish that the pair is finite depth if and only if Q is an algebraic module in the representation ring of either H or R. A necessary condition for finite depth is provided as the stabilisation of the descending chain of annihilators of the tensor powers of Q. In Danz, Comm of Alg, 39:5, 1635- 1645, (2011) the author provides a formula for the depth of a complex twisted group extension of the symmetric groups of order n and n + 1. We provide a general setting in which the depth of the complex crossed product algebra extension D#H D#G of a group pair H < G is always less or equal than the depth of the algebra extension kH < kG. For this we use the entwining of a left H-module algebra A with an H-module coalgebra C, over a Hopf algebra H. We show that such a structure is a Galois coring when A = H and C = Q for a finite dimensional Hopf algebra extension R H, and that this extends to the crossed product algebra extension of a finite group extension. We also provide a setting for the depth of factorisation algebras and provide a formula for the value of depth of a subalgebra of a factorisation algebra in terms of its module depth.
The Short Course is part of the Course on Algebra and Categories of the Summer School of the UC|UP Joint PhD Program in Mathematics. It is organized by Professor Manuel Delgado (mdelgado@fc.up.pt). It will be 7.5 hours long (1.5h per day).
Short Course on Categorical Algebra
Lecturer: Professor Marino Gran, Université Catholique de Louvain
Department of Mathematics, University of Coimbra
September 12, 2016 -
September 16, 2016
The Short Course is part of the Course on Algebra and Categories of the Summer School of the UC|UP Joint PhD Program in Mathematics. It is organized by Professor Maria Manuel Clementino (mmc@mat.uc.pt). It will be 7.5 hours long (1.5h per day).
The Short Course is part of the Course on Dynamics and Optimization of the Summer School of the UC|UP Joint PhD Program in Mathematics. It is organized by Professor Manuela Aguiar (maguiar@fep.up.pt). It will be 7.5 hours long (1.5h per day).
The Short Course is part of the Course on Dynamics and Optimization of the Summer School of the UC|UP Joint PhD Program in Mathematics. It is organized by Professor Joao Gouveia (jgouveia@mat.uc.pt). It will be 7.5 hours long (1.5h per day).
PhD Defense
Fatemeh Esmaeili Taheri - Numerical ranges of linear pencils
Sala dos Capelos, Universidade de Coimbra
July 27, 2016
In recent years, the numerical range of finite matrices and linear operators has been intensively investigated. In this thesis, the concept of numerical range of a linear pencil is discussed, and the geometry of the numerical range is investigated by using techniques of plane algebraic geometry. The classification of all possible boundary generating curves of the numerical range of pencils of two-by-two and three-by-three matrices is explicitly given, when one of the matrices is hermitian. The numerical range of linear pencils with hermitian coefficients has been studied by some authors. We have characterized the numerical range of self-adjoint linear pencils, pointing out and correcting an error reproduced in the literature. For the case n=2 , the boundary generating curves of numerical range are conics. Geometrical proofs of the Elliptical Range Theorem, Parabolical Range Theorem and Hyperbolical Range Theorem, have been obtained in an unified way. We remark that the two-by-two case is particularly important, since for a pencil of arbitrary dimension the compression to the bidimensional case gives us information on the general n by n case. For n = 3, we obtained the classification of all possible boundary generating curves of the numerical range, distinguishing the case of one of the matrices being positive (negative) definite, semidefinite and indefinite. All the possible boundary generating curves of the numerical range of three-by-three linear pencils can be completely described by using Newton’s classification of cubic curves. The obtaining results are illustrated by numerical examples.
Count time series modeling has drawn much attention and considerable development in the recent decades since many of the observed stochastic systems in various contexts and scientific fields are driven by such kind of data. The first modelings, with linear character and essentially inspired by the classic ARMA models, are proved to be insufficient to give an adequate answer for some empirical characteristics, also observed in this type of data, such as the conditional heteroscedasticity. In order
to capture such kind of characteristics several models for nonnegative integer-valued time series arise in literature inspired by the classic GARCH model of Bollerslev [10], among which is highlighted the integer-valued GARCH model with conditional Poisson distribution (briefly INGARCH model), proposed in 2006 by Ferland, Latour and Oraichi [25].
The aim of this thesis is to introduce and analyze a new class of integer-valued models having an analogous evolution as considered in [25] for the conditional mean, but with an associated comprehensive family of conditional distributions, namely the family of infinitely divisible discrete laws with support in N0, inflated (or not) in zero. So, we consider a family of conditional distributions that in its more general form can be interpreted as a mixture of a Dirac law at zero with any discrete
infinitely divisible law, whose specification is made by means of the corresponding characteristic function. Taking into account the equivalence, in the set of the discrete laws with support N0, between infinitely divisible and compound Poisson distributions, this new model is designated as zero-inflated compound Poisson integer-valued GARCH model (briefly ZICP-INGARCH model).
We point out that the model is not limited to a specific conditional distribution; moreover, this model has as main advantage to unify and enlarge substantially the family of integer-valued stochastic processes. It is stressed that it is possible to present new models with conditional distributions with interest in practical applications as, in particular, the zero-inflated geometric Poisson INGARCH and the zero-inflated Neyman type-A INGARCH models, and also recover recent contributions such as the
(zero-inflated) negative binomial INGARCH [81, 84], (zero-inflated) INGARCH [25, 84] and (zeroinflated) generalized Poisson INGARCH [52, 82] models. In addition to having the ability to describe different distributional behaviors and consequently, different kinds of conditional heteroscedasticity, the ZICP-INGARCH model is able to incorporate simultaneously other stylized facts that have been recorded in real count data, in particular overdispersion and high occurrence of zeros.
The probabilistic analysis of these models, concerning in particular the development of necessary and sufficient conditions of different kinds of stationarity (first-order, weak and strict) as well as the property of ergodicity and also the existence of higher order moments, is the main goal of this study. It is still derived estimates for the parameters of the model using a two-step approach which is based on the conditional least squares and moments methods.
Eilenberg and Steenrod proved that ordinary homology is characterized by five axioms. Later, Atiyah, Hirzebruch and Whitehead observed that there are other families of functors that satisfy the four "most important" axioms. They defined the so called "generalized homology theories" (or "homology theories") which are examples of stable phenomena in homotopy theory. The concept of a prespectrum was first introduced by Elon Lages Lima in his PhD thesis to study some kinds of stable phenomena, such as Spanier-Whitehead duality and Stable Postnikov invariants. Later, Adams and Boardman proposed the first homotopy category of pre spectrums This was the starting point of the research field called stable homotopy theory. Nowadays stable homotopy categories are fundamental for studying all kind of stable phenomena in homotopy theory, including generalized homology theories and cohomology theories. The goal of the talk is to present some basic results of algebraic topology and give some elementary stable (and unstable) results of homotopy theory. To reach this goal, we shall introduce the concept of derived functors, homotopy colimits and assume two basic theorems: homotopy excision and long exact sequence of homotopy groups. At the end, we shall prove that every prespectrum represents a homology theory.
(*) Fernando Lucatelli Nunes is a PhD student for the Joint PhD Program in Mathematics UC|UP working at the University of Coimbra in the area of Algebra, Logic and Topology under the supervision of professor Maria Manuel Clementino.
Homological Algebra
Artur de Araujo (UP student)
15h, Room 5.5, UC Math. Dept. (Research Seminar Program)
June 28, 2016
We will explain the basic concepts of Homological Algebra (Cech cohomology, injective/projective resolutions, derived functors,) and show why, useful as they are, they have shortcomings for a general theory of cohomology. Time allowing, we'll give a hint of why derived categories make up for those defficiencies.
(*) Artur de Araujo is a PhD student for the Joint PhD Program in Mathematics UC|UP working at the University of Porto in the area of Geometry and Topology under the supervision of professor Peter Gothen.
Topological spaces as algebras
Pier Giorgio Basile (UC student)
14h, Room M004, UP Math. Dept. (Research Seminar Program)
May 19, 2016
Monads, or triples, and the algebras they define, proved to be very important in several fields of mathematics. For example, they allow us to see the algebraic nature owned by topological spaces. This fact, for what concerns compact Hausdorff spaces, is known since 1969 (see [4]). By suitably weakening the axioms of the algebras, M. Barr in 1970 (see [1]) generalized the result to all topological spaces. This step represented a starting point of the new theory of lax (Eilenberg-Moore) algebras, introduced about thirty years later by M.M. Clementino, D. Hofmann and W. Tholen (see [2] and [3]).
[1] M. Barr, Relational Algebras, in: Reports of the Midwest Category Seminar, IV, pp 39-55, Lecture Notes in Mathematics 137, Springer, Berlin (1970).
[2] M.M. Clementino and D. Hofmann, Topological features of lax algebras, Appl. Categ. Structures 11 (2003), 267-286.
[3] M.M. Clementino and W. Tholen, Metric, topology and multicategory - a common approach, J. Pure Appl. Algebra 179 (2003), 13-47.
[4] E. Manes, A triple theoretic construction of compact algebras, 1969 Sem. on Triples and Categorical Homology Theory (ETH Zurich 1966/67), 91118, Lecture Notes in Mathematics, Springer, Berlin.
(*) Pier Giorgio Basile is a student for the Joint PhD Program in Mathematics UC|UP working at University of Coimbra in the area of "Algebra, Logic and Topology" under the supervision of Prof. Maria M. Clementino.
PhD Defense
Célia Borlido - The word problem and some reducibility properties for pseudo-varieties of the form DRH
Univ. Porto
April 27, 2016
On topological semi-abelian algebras
Mathieu Duckerts-Antoine (CMUC, Portugal)
14h, Room 5.5, Department of Mathematics, University of Coimbra
April 7, 2016
In this talk, we will study some aspects of the categories of topological semi-abelian algebras. In particular, I will explain why these categories are homological. If the time allows it, I will also explain what is a torsion theory in a homological category and give several examples in the context under consideration.
References:
- F. Borceux and M. M. Clementino, Topological semi-abelian algebras, Advances in Mathematics, 190 (2005), 425-453
- D. Bourn and M. Gran, Torsion theories in homological categories, Journal of Algebra, 305 (2006), 18-47.
The k-word problem over DRG
Célia Borlido (UP student)
14h, Room 5.5, UC Math. Dept. (Research Seminar Program)
March 10, 2016
The study of finite semigroups has its roots in Theoretical Computer Science. In particular, in the mid nineteen seventies, Eilenberg [2] established the link between "varieties of rational languages", which is an important object of study in Computer Science, and certain classes of finite semigroups, known as "pseudo varieties". At the level of pseudovarieties, some problems arise naturally, one of them being the so-called "word problem". Roughly speaking, it consists in deciding whether two expressions define the same element in every semigroup of a given pseudovariety.In this talk, we start by introducing some basic background on finite semigroups. Our first goal is to explain what the "k-word problem over DRG" is about. After that, based on some illustrative examples, we intend to give intuition on how to show that the referred problem is decidable. Our solution extends work of Almeida and Zeitoun [1] on the pseudovariety consisting of all "R-trivial semigroups".
References:
[1] J. Almeida and M. Zeitoun, An automata-theoretic approach to the word problem for ?-terms over R, Theoret. Comput. Sci. 370 (2007), no. 1-3, 131-169.
[2] S. Eilenberg, Automata, languages, and machines. Vol. B, Academic Press, New York - London, 1976.
(*) Célia Borlido is a student for the Joint PhD Program in Mathematics UC|UP working at the University of Porto in the area of “Semigroups, Automata and Languages” under the supervision of Prof. Jorge Almeida.
PhD Defense
Azizeh Nozad - Hitchin Pairs for indefinite unitary Group
Room 107, Dep. Mathematics, Univ. Porto
February 26, 2016
Besov and Triebel-Lizorkin Spaces with Variable Exponents
Helena Gonçalves (Technische Univ. Chemnitz, Germany)
14h, Room M030, UP Math. Dept. (Research Seminar Program)
February 18, 2016
After an introduction on classical function spaces, we introduce spaces of Besov and Triebel-Lizorkin type B^{s}_{p,q}(R^{n}) and F^{s}_{p,q}(R^{n}) by Fourier analytical methods and present some properties of those spaces.
Thereafter, we step up to the scale of function spaces with variable exponents, mainly the variable Lebesgue space L_{p(.)}(R^{n}). With this space in mind, we introduce two generalizations of B^{s}_{p,q}(R^{n}) and F^{s}_{p,q}(R^{n}): Besov and Triebel-Lizorkin spaces with variable smoothness and integrability B^{s(.)}_{p(.),q(.)}(R^{n}) and F^{s(.)}_{p(.),q(.)}(R^{n}), and 2-microlocal Besov and Triebel-Lizorkin spaces B^{w(.)}_{p(.),q(.)}(R^{n}) and F^{w(.)}_{p(.),q(.)}(R^{n}). We focus our attention on the last scale, where some properties will be considered.
Helena Gonçalves is working as a Research Assistant at Chemnitz University of Technology, Germany in the area of "Analysis" under the supervision of Prof. Henning Kempka.
The spectral inclusion regions of linear pencils and numerical range
Fatemeh Esmaeili Taheri (UC student)
14h, Room M030, UP Math. Dept. (Research Seminar Program)
January 28, 2016
Let A,B be n×n (complex) matrices. We are mainly interested in the study of the structure of the spectrum of a linear pencil, that is, a pencil of the form A-?B, where λ is a complex number. Our main purpose is to obtain spectral inclusion regions for the pencil based on numerical range. The numerical range of a linear pencil of a pair (A, B) is the set W(A,B) = {x*(A-λB)x : x \in Cn, |x| = 1, λ \in C}. The numerical range of linear pencils with hermitian coefficients was studied by some authors. We are mainly interested in the study of the numerical range of a linear pencil, A -λB, when one of the matrices A or B is Hermitian and λ \in C. We characterize it for small dimensions in terms of certain algebraic curves. The results are illustrated by numerical examples.
Fatemeh Esmaeili Taheri is a student of the Joint PhD Program in Mathematics UC|UP working at University of Coimbra in the area of "Algebra and Combinatorics" under the supervision of Prof. Natália Bebiano.
An Overview on the Dimension of Projected Patterns in Reaction-Diffusion Systems
Juliane Fonseca de Oliveira (UP student)
12h, Room 5.5, UC Math. Dept. (Research Seminar Program)
December 17, 2015
In the study of pattern formation in symmetric physical systems a 3-dimensional structure in thin domains is often modeled as 2-dimensional one. As a contrast, in this work we use the full 3-dimensionality of the problem to give a theoretical interpretation and possibly decide whether the pattern seen in Reaction Diffusion systems naturally occur in either 2- or 3- dimension. For this purpose, we are concerned with functions in R3 that are invariant under the action of a crystallographic group and the symmetries of their projections into a function defined on a plane.
Tuning Polymeric and Drug Properties in a Drug-Eluting Stent: A Numerical Study
Jahed Naghipoor (Institute of Structural Mechanics (ISM), Bauhaus-Universität Weimar)
14h30, Room 5.5, UC Math. Dept. (Research Seminar Program)
December 17, 2015
In recent years, mathematical modeling of cardiovascular drug delivery systems has become an effective tool to gain deeper insights in the cardiovascular diseases like atherosclerosis. In the case of the coronary biodegradable stent, it leads to a deeper understanding of drug release mechanisms from polymeric stent into the arterial wall. In this talk, a two-dimensional coupled nonlinear non-Fickian model for drug release from a biodegradable drug-eluting stent into the arterial wall is presented. The influence of porosity and degradation of the polymer as well as the dissolution rate of the drug are analyzed. Numerical simulations that illustrate the kind of dependence of drug profiles on these properties are included.
The Electrostatic Limit for the Zakharov System
Luigi Forcella (Scuola Normale Superiore di Pisa, Italy)
14h, Room M031, UP Math. Dept. (Research Seminar Program)
November 19, 2015
The Zakharov system describes the coupled dynamics of the electric field amplitude and the low frequency fluctuation of the ions in a unmagnetized or weakly magnetized plasma. This system couples Schrödinger-like and wave equations and in its physical derivation depends on a parameter $\alpha.$ Large value of $\alpha$ describes a plasma that is very hot, so it is meaningful to study the limit for the solutions to this system as $\alpha$ goes to infinity. In this talk we give rigorous mathematical result in this direction.
The speaker is a PhD student at Scuola Normale Superiore di Pisa, Italy and working in the area of "Analysis" under the supervision of Prof. Luigi Ambrosio.
Analogies between Optimal Transport and Minimal Entropy
Luigia Ripani (Univ. Claude Bernard Lyon 1, France)
14h30, Room M031, UP Math. Dept. (Research Seminar Program)
November 19, 2015
The Schrödinger problem is an entropic minimization problem and it's a regular approximation of the Monge-Kantorovich problem, at the core of the Optimal Transport theory.
In this talk I will first introduce the two problems, then I will describe some analogy between optimal transport and the Schrödinger problem such as a dual Kantorovich type formulation, the dynamical Benamou-Brenier type representation formula, as well as a characterization formula and some properties of the respective solutions.
Finally I will mention, as an application of these analogies, some contraction inequalities with respect to the entropic cost, instead of the classical Wasserstein distance.
The speaker is a PhD student at Institut Camille Jordan - Université Claude Bernard Lyon 1, France and working in the area of "PDE, Analysis" under the supervision of Prof. Ivan Gentil and Prof. Christian Léonard.
On Pseudovarieties of Forest Algebras
Saeid Alirezazadeh (UP student)
15h, Room 5.5, UC Math. Dept. (Research Seminar Program)
October 27, 2015
Forest algebras are used in the theory of formal languages. They consist of two monoids, the horizontal one H and the vertical one V , with an action of V on H, and a complementary axiom of faithfulness. The main example is the forest algebras of plane forests and contexts, that is to say plane forests with a deleted leaf, which is the free object of the theory. In the study of forest algebras one of the main difficulty is how to handle the faithfulness property. A pseudovariety is a class of finite algebras of a given signature, closed under the taking of homomorphic images, subalgebras and finitary direct products.
A profinite algebra is defined to be a projective limit of a projective system of finite algebras. We tried to adapt in this context some of the results in the theory of semigroups, specially the results on relatively free profinite semigroups which have been shown to be an important tool in the study of pseudovarieties of semigroups.
We recall definition of forest algebras and state several basic results concerning definition and properties of forest algebras and the free forest algebra which are used later on.
Bojanczyk and Walukiewicz in ["Forest algebras", Logic and Automata, 2008, pp. 107-131] defined the syntactic forest algebra over a forest language. We define a new version of syntactic congruence of a subset of the free forest algebra, not just a forest language, which is used in the proof of an analog of Hunter's Lemma ["Certain finitely generated compact zero-dimensional semigroups", 1988, pp. 265-270]. The new version of syntactic congruence is the natural extension of the syntactic congruence for monoids in case of forest algebras. We show that for an inverse zero action subset and a forest language which is the intersection of the inverse zero action subset with the horizontal monoid, the two versions of syntactic congruences coincide.
Almeida in ["Profinite semigroups and applications", Structural Theory of Automata, Semigroups, and Universal Algebra, 2005, pp 1-45] established some results on metric semigroups. We adapted some of his results to the context of forest algebras. We define on the free forest algebra a pseudo-ultrametric associated with a pseudovariety of forest algebras. We show that the analog of Hunter's Lemma holds for metric forest algebras, which leads to the result that zero-dimensional compact metric forest algebras are residually finite. We show an analog of Reiterman's Theorem ["The Birkhoff theorem for finite algebras", 1982, pp. 1-10], which is based on a study of the structure profinite forest algebras.
Rolling Maps and Applications
Maria de Fátima Alves de Pina (UC student)
14h, Room M031, UP Math. Dept. (Research Seminar Program)
October 22, 2015
Rolling motions are rigid motions subject to holonomic and nonholonomic constraints. These motions appear associated to certain engineering areas, such as robotics and computer vision. Rolling maps are the mathematical tools to describe rolling motions.
In this talk, the concept of rolling map in a Riemannian framework will be presented together with some properties and applications. From the nonholonomic constraints of no-slip and no-twist the kinematic equations of motion can be derived. This will be done for the rolling of some particular manifolds that play an important role in applications. Explicit solutions of the kinematic equations will be derived when the manifolds roll along geodesics.
Let X be a Riemann surface of genus g greater or equal than 2. A twisted U(p,q)-Higgs bundle consists of a pair of holomorphic vector bundles on a Riemann surface, together with a pair of twisted maps between them. Here we study the variation with the parameter of the moduli space of twisted U(p,q)-Higgs bundles with a view to obtaining birationality results.
Numerical Solution of Time-Dependent Maxwells Equations in Anisotropic Materials for Modelling Light Scattering in Human Eyes Structure
Maryam Khaksar Ghalati (UC student)
14h, Room M031, UP Math. Dept. (Research Seminar Program)
September 24, 2015
Modelling light propagation in biological tissue has become an important research topic in biomedical optics with application in diverse fields as for example in ophthalmology. Waveguides with induced anisotropy may worth to be modeled as they could play a role in biological waveguides. For instance, there is a strong correlation between retinal nerve fiber layer thinning and reduction in tissue birefringence.
Research and Events
Defended Theses
Structure, algorithmics and dynamics of endomorphisms for certain classes of groups
André da Cruz Carvalho (April 2023)
Pedro V. Silva
A point-free study of z-embeddings, more general classes of localic maps, and uniform continuity
Ana Belén Avilez García (April 2023)
Jorge Picado
Analysis of equations of motion of inextensible strings and networks