- Tingueu el micròfon apagat tota l'estona excepte durant el "Cafè i galetes".
- Podeu tenir la càmera encesa si voleu, però en cas de saturaciò de línies, caldrà que la tanqueu.
- Si voleu fer una pregunta, si us plau demaneu torn a través del xat i espereu que els moderadors us donin la paraula.
- Si és possible, entreu amb antelació. Els que no useu un compte de correu de la UPC haureu d'esperar a ser admesos per accedir-hi.

Dia: Dimecres, 14 de setembre de 2022

Lloc: Aula S04, Facultat de Matemàtiques i Estadística, UPC. Pau Gargallo,14 BCN.

També ONLINE https://meet.google.com/zvg-pajn-owr

- Hora: 16h00m

A càrrec de: Jaume Paradela (UPC)

Títol: Unstable Motions in the Restricted Planar Elliptic Three Body Problem

Resum: Consider the restricted planar elliptic 3 body problem (RPE3BP), where a massless body moves on the gravitational field generated by two bodies, called primaries, which have masses *μ∈(0,1/2]* and *1-μ* and revolve one around each other in Keplerian ellipses of eccentricity *ε∈(0,1)*. We prove that the RPE3BP exhibits unstable dynamics: for any value *μ∈ (0,1/2)*, we build orbits along which the angular momentum *G* of the massless body experiences an arbitrarily large variation provided the eccentricity of the primaries is small enough. This kind of unstable motion is also referred to as Arnold diffusion in the literature.

The proof relies on the existence of a transition chain of heteroclinic orbits to a Topological Normally Hyperbolic Invariant Cylinder (TNHIC). Two main sets of tools are developed to construct this diffusive mechanism in the RPE3BP. First, for the study of highly anisotropic splitting (involving exponentially and non exponentially small directions) between the invariant manifolds of pairs of partially hyperbolic invariant tori. Second, for the analysis of two distinct scattering maps associated to two different transversal intersections of the invariant manifolds of a (Topological) NHIC. In particular, for comparing them when they are exponentially close to each other. This is a joint work with Marcel Guardia and Tere Seara.

Dia: Dimecres, 28 de setembre de 2022

Lloc: Aula B1 (planta baixa), Facultat de Matemàtiques i Informàtica, UB.

També ONLINE https://ub-edu.zoom.us/j/99202881559?pwd=cDBlY3d4TS9Xam56bXVIU3c1TmtRUT09

- Hora: 16h00m

A càrrec de: Pablo Roldán (Yeshiva University)

Títol: Continuation of relative equilibria in the n–body problem to spaces of constant curvature

Resum: The curved n-body problem is a natural extension of the planar Newtonian n-body problem to surfaces of non-zero constant curvature. We prove that all non-degenerate relative equilibria of the planar problem can be continued to spaces of constant curvature κ, positive or negative, for small enough values of this parameter. We also compute the extension of some classical relative equilibria to curved spaces using numerical continuation. For example, we extend Lagrange’s triangle configuration with different masses to both positive and negative curvature spaces.

Dia: Dimecres, 19 d'octubre de 2022

Lloc: Aula S04, Facultat de Matemàtiques i Estadística, UPC. Pau Gargallo,14 BCN.

També ONLINE https://meet.google.com/zvg-pajn-owr

- Hora: 16h00m

A càrrec de: Alexey Kazakov (HSE University Nizhny Novgorod)

Títol: Four-winged and two-winged Lorenz attractors emerge due to bifurcations of a periodic orbit with multipliers *(-1,i,-i)*

Resum: We show that bifurcations of periodic orbits with multipliers *(-1,i,-i)* can lead to the birth of pseudohyperbolic Lorenz-like attractors of two different types: one is a discrete analogue of the classical Lorenz attractor and the other is new, a four-winged Simó angel. We also show that these two attractors exist for an orientation-reversing, quadratic, three-dimensional Hénon map, which implies the abundance of such attractors in a class of systems with homoclinic tangencies. The analysis is based on the study of a normal form for this bifurcation, a slow-fast three-dimensional system of differential equations with a Z4 symmetry. The existence of the continuous-time counterparts of the discrete 2- and 4-winged Lorenz-like attractors is established for the normal form as a part of an extensive numerical and theoretical analysis of its bifurcations. In particular, we establish that both 2- and 4-winged continuous-time Lorenz attractors are born out of a certain Z4-symmetric heteroclinic configurations with 3 saddles.

Dia: Dimecres, 26 d'octubre de 2022

Lloc: Aula T2 (segon pis), Facultat de Matemàtiques i Informàtica, UB.

També ONLINE https://ub-edu.zoom.us/j/99202881559?pwd=cDBlY3d4TS9Xam56bXVIU3c1TmtRUT09

- Hora: 16h00m

A càrrec de: Donato Scarcella, Paris-Dauphine University

Títol: Asymptotically quasiperiodic solutions for time-dependent Hamiltonians

Resum: In 1954 Kolmogorov laid the foundation for the so-called KAM theory. This theory shows the persistence of quasiperiodic solutions in nearly integrable Hamiltonian systems. It is motivated by classical problems in celestial mechanics, such as the n-body problem.

In this talk, we are interested in perturbations which depend on time non-quasiperiodically. We will analyze some properties of time-dependent Hamiltonians converging, when time tends to infinity, to autonomous Hamiltonians having an invariant torus supporting quasiperiodic solutions. We will look for solutions converging, asymptotically in time, to the quasiperiodic solutions of the unperturbed autonomous system.

Moreover, we will analyze the example in celestial mechanics of a planetary system perturbed by a given comet coming from and going back to infinity, asymptotically along a hyperbolic Keplerian orbit.

Dia: Dimecres, 2 de novembre de 2022

Lloc: Aula S04, Facultat de Matemàtiques i Estadística, UPC. Pau Gargallo,14 BCN.

També ONLINE https://meet.google.com/zvg-pajn-owr

- Hora: 16h00m

A càrrec de: Renato Calleja (IIMAS-UNAM, Universitat de Barcelona)

Títol: Quasi-periodic attractors up to the breakdown in the spin-orbit problem

Resum: We consider the dissipative spin-orbit problem in Celestial Mechanics, which describes the rotational motion of a triaxial satellite moving on a Keplerian orbit subject to tidal forcing and drift. This problem is an example of a conformally symplectic system, which is characterized by the property to transform the symplectic form into a multiple of itself. We construct and continue quasi-periodic solutions with fixed frequency, satisfying appropriate conditions. The construction is based on a KAM theorem for conformally symplectic systems, which also provides estimates on the breakdown threshold of the invariant attractor. To construct the invariant attractor, we will use high precision numerical simulations to compute some of the required quantities. The algorithms are guaranteed to reach arbitrarily close to the border of existence, given enough computer resources. This talk refers to joint works with A. Celletti, J. Gimeno and R. de la Llave.

Dia: Dimecres, 9 de novembre de 2022

Lloc: Aula T2 (segon pis), Facultat de Matemàtiques i Informàtica, UB.

- Hora: 16h00m

A càrrec de: Andrew Clarke, Universitat de Barcelona

Títol: Why are inner planets not inclined?

Resum: Consider the 4-body problem with arbitrary masses in the regime where 3 bodies revolve around the other. We assume that the semimajor axes of the orbital ellipses are of different orders, and that there is non-negligible mutual inclination between the orbital planes of bodies 1 and 2. We prove that, given any finite itinerary of the angular momentum vector of body 2, there exist orbits of the 4-body problem shadowing this itinerary with arbitrary precision. From a geometric point of view, this implies that the eccentricity of the orbit of body 2 and the mutual inclination of the orbital planes of bodies 2 and 3 can be made to follow any finite itinerary. For example, the second planet can flip from prograde to retrograde nearly horizontal revolutions and back again arbitrarily many times.

Dia: Dimecres, 16 de novembre de 2022

Lloc: Aula S04, Facultat de Matemàtiques i Estadística, UPC. Pau Gargallo,14 BCN.

- Hora: 16h00m

A càrrec de: Makrina Agaoglou (ICMAT)

Títol: Transport processes and chemical reaction dynamics: The case of a Potential Energy Surface with four wells and an index-2 saddle

Resum: In this talk, we will explore the phase space structures governing isomerization dynamics on a potential energy surface with four wells and an index-2 saddle. For this model, we analyze the influence that coupling both degrees of freedom of the system and breaking the symmetry of the problem have on the geometrical template of phase space structures that characterizes reaction, using the method of Lagrangian Descriptors.

*Esta charla está parcialmente soportada con la Beca Leonardo a Investigadores y Creadores Culturales 2022 de la Fundación BBVA. La Fundación BBVA no se responsabiliza de las opiniones, comentarios y contenidos incluidos en este seminario los cuales son total y absoluta responsabilidad de sus autores.*

Dia: Dimecres, 23 de novembre de 2022

Lloc: Aula T2 (segon pis), Facultat de Matemàtiques i Informàtica, UB.

- Hora: 16h00m

A càrrec de: Marina Vegué, Universitat Politècnica de Catalunya

Títol: Firing rate and synaptic weight distributions in plastic networks of spiking neurons

Resum: Networks of spiking neurons have been widely used as models to represent neuronal activity in the brain. These models are reasonably realistic but they are also difficult to treat analytically. Mean-field theory has nevertheless proven to be successful as a method for deriving some of their statistical properties at equilibrium such as the distribution of firing rates. However, these models assume a static connectivity, whereas the connection strengths in real brain networks evolve in time according to plasticity rules that depend on the neuronal activity.

In this talk I will present a way to extend the mean-field formalism to networks with synaptic weights that are prone to plastic, activity-dependent modulation. The plasticity in the model is mediated by the introduction of spike traces. A trace associated to one neuron represents a chemical signal that is released every time the neuron emits a spike and which is degraded over time. The temporal evolution of the trace is controlled by its degradation rate (i.e., a measure of how fast the spiking "memory" is lost) and by the neuron's firing rate.

The mean-field formalism provides a set of equations whose solution specifies the firing rate and synaptic weight distributions at equilibrium. The equations are exact in the limit of traces with infinite memory but I will show that they already provide accurate results when the degradation rate lies within the physiological range. Overall, this work offers a way to explore and better understand the way in which plasticity shapes both activity and structure in neuronal networks.

Dia: Dimecres, 14 de desembre de 2022

Lloc: Aula T2 (segon pis), Facultat de Matemàtiques i Informàtica, UB.

- Hora: 16h00m

A càrrec de: Michel Orieux, Universitat Politècnica de Catalunya

Títol: Control theory for selective neuronal communication.

Resum: On many scales in the brain, from single neurons to populations of tens of thousands and even in whole brain models, oscillatory behaviors can be scouted. In recent years, many tools from dynamical systems have been developed in order to study these oscillators. In this talk, I will expose how to take advantage of this work, and advances in optimal control theory to establish selective communications between populations of neurons modeled by a set of differential equations.

Dia: Dimecres, 11 de gener de 2023

Lloc: Aula S04, Facultat de Matemàtiques i Estadística, UPC. Pau Gargallo,14 BCN.

- Hora: 16h00m

A càrrec de: Francisco Marques, Universitat Politècnica de Catalunya

Títol: Extensional channel flow: a beautiful example for dynamical systems theory.

Resum: Extensional self-similar flows in a channel are explored numerically for arbitrary stretching-shrinking rates of the confining parallel walls. The present analysis embraces time integrations, and continuations of steady and periodic solutions unfolded in the parameter space. We have adopted a dynamical systems perspective, analysing the instabilities and bifurcations the base state undergoes when increasing the Reynolds number. It has been found that the base state becomes unstable for small Reynolds numbers, and a transitional region including complex dynamics takes place at intermediate Reynolds numbers, depending on the wall acceleration values. The base flow instabilities are constitutive parts of different codimension-two bifurcations that control the dynamics in parameter space.

Dia: Dimecres, 18 de gener de 2023

Lloc: Aula T2 (segon pis), Facultat de Matemàtiques i Informàtica, UB.

- Hora: 16h00m

A càrrec de: Anna Florio (Université Paris Dauphine-PSL)

Títol: Universal dynamics in 3D stationary Euler flows

Resum: Understanding the dynamical complexity of an ideal fluid has motivated many research works. According to Arnold's vision, the dynamics of stationary Euler flows should be as complicated as those in celestial mechanics. In a joint work with Pierre Berger and Daniel Peralta-Salas, we recover Arnold's vision by showing the existence of a locally dense set of stationary solutions of the Euler equations in ℝ³ made up of universal vector fields. For the proof we introduce new perturbative methods in the context of Beltrami fields in order to import tools from bifurcation theory.

Dia: Dimecres, 15 de febrer de 2023

Lloc: Aula T2 (segon pis), Facultat de Matemàtiques i Informàtica, UB.

- Hora: 16h00m
- Hora: 17h00m

A càrrec de: Kostiantyn Drach, Institute of Science and Technology Austria (ISTA).

Títol: Rigidity of rational maps

Resum: In one-dimensional complex dynamics, a branch of dynamical systems that studies iterations of holomorphic maps, the rigidity question has the classical form: Under which conditions can one promote topological conjugacy between a pair of maps to an analytic (conformal) conjugacy? We call this 'parameter rigidity', as it allows us to distinguish maps within parameter space starting with some 'soft' topological (or even combinatorial) data. On the other hand, for a given map, there is a parallel 'dynamical rigidity' question: Can one distinguish individual orbits in combinatorial terms (e.g. via symbolic dynamics)? For polynomials, and especially for quadratic polynomials, this circle of questions is well-studied in the works of Avila, Douady, Hubbard, Lyubich, McMullen, Sullivan, van Strien, Yoccoz, and many others. In this case, the progress was possible thanks to the fact that most polynomials have a naturally defined Markov partition in their dynamical plane (via so-called Yoccoz puzzles). This is not true for general rational maps, and hence life becomes more complicated (or interesting?). In my talk, I will discuss what we know so far about dynamical and parameter rigidity of general rational maps. Our guiding example will be Newton maps, a family of maps naturally arising from Newton's root-finding method. I will also outline a 'toolbox' of techniques useful to attack these types of questions; the key 'tool' is a renormalization concept of complex box mapping. The main message of the talk hopefully will be that life is not that complicated outline a 'toolbox' of techniques useful to attack these types of questions; the key 'tool' is a renormalization concept of complex box mapping. The main message of the talk hopefully will be that life is not that complicated after all (under certain conditions).

A càrrec de: Stefanella Boatto, Dept of Applied mathematics, Institute of Mathematics, Federal University of Rio de Janeiro

Títol: A journey into the mathematical universe: Topological Data Analysis. A brief introduction and some applications.

Resum: Data Science is a highly evolving discipline that presents many challenges. For example, in the case of an epidemic, how can we identify which socio-economic and social parameters, tourist mobility, prevention and control measures have the greatest impact on the dynamics of an epidemic? In recent years Topological Data Analysis has proved to be a powerful tool, a lens to highlight some dominant features of collective phenomena and explore large and complex data sets. In this lecture we will see a brief introduction and how this new discipline is highly multidisciplinary within mathematics itself, bringing together areas such as Space-Time Analysis, Algebraic Topology, Complex Networks, Alg. Linear and Computation.

Dia: Dimecres, 22 de febrer de 2023

Lloc: Aula S04, Facultat de Matemàtiques i Estadística, UPC. Pau Gargallo,14 BCN.

- Hora: 16h00m

A càrrec de: Jaime Paradela (UPC)

Títol: Oscillatory motions in the Restricted 3 Body Problem: A functional analytic approach.

Resum: A fundamental question in Celestial Mechanics is to analyze the possible final motions of the Restricted 3-body Problem, that is, to provide a qualitative description of its complete (i.e. defined for all time) orbits as time goes to infinity. According to the classification given by Chazy back in 1922, a remarkable possible behaviour is that of oscillatory motions, where the motion of the massless body is unbounded but returns infinitely often inside some bounded region. In contrast with the other possible final motions in Chazy’s classification, oscillatory motions do not occur in the 2-body Problem, while they do for larger numbers of bodies. A further point of interest is their appearance in connection with the existence of choaotic dynamics. In this paper we introduce new tools to study the existence of oscillatory motions and prove that oscillatory motions exist in a particular configuration known as the Restricted Isosceles 3-body Problem (RI3BP) for almost all values of the angular momentum. Our method, which is global and not limited to nearly integrable settings, extends previous results by Guardia, Paradela, Seara and Vidal by blending variational and geometric techniques with tools from nonlinear analysis such as topological degree theory. To the best of our knowledge, the present work constitutes the first complete analytic proof of existence of oscillatory motions in a non perturbative setting.

This is joint work with Susanna Terracini.

Dia: Dimecres, 1 de març de 2023

Lloc: Aula T2 (segon pis), Facultat de Matemàtiques i Informàtica, UB.

- Hora: 16h00m

A càrrec de: Chongchun Zeng, Georgia Institute of Technology

Títol: Capillary Gravity Water Waves Linearized at Monotone Shear Flows: Eigenvalues and Inviscid Damping

Resum: We consider the 2-dim capillary gravity water wave problem - the free boundary problem of the Euler equation with gravity and surface tension - of finite depth *x₂∈ (-h,0)* linearized at a uniformly monotonic shear flow *U(x₂)*. Our main focus are eigenvalue distribution and inviscid damping. We first prove that in contrast to finite channel flow and gravity waves, the linearized capillary gravity wave has two unbounded branches of eigenvalues for high wave numbers. They may bifurcate into unstable eigenvalues through a rather degenerate bifurcation. Under certain conditions, we provide a complete picture of the eigenvalue distribution. Assuming there are no singular modes (i.e. embedded eigenvalues), we obtain the linear inviscid damping. We also identify the leading asymptotic terms of the velocity and obtain stronger decay for the remainders.

This is a joint work with Xiao Liu.

Dia: Dimecres, 8 de març de 2023

Lloc: Aula S04, Facultat de Matemàtiques i Estadística, UPC. Pau Gargallo,14 BCN.

- Hora: 16h00m

A càrrec de: Frank Trujillo (Universität Zürich)

Títol: Ejemplos de inestabilidad para hamiltonianos con dos grados de libertad.

Resum: Es sabido que los puntos fijos elípticos genéricos de hamiltonianos suaves con dos grados de libertad son estables en el sentido de Lyapunov. Más concretamente, son KAM-estables, es decir, acumulados por toros invariantes cuya densidad de Lebesgue tiende a 1 cerca del punto fijo, e isoenergéticos no degenerados, lo que garantiza que cada superficie de energía contiene alguno de los toros invariantes. Se sabe que estas dos condiciones implican la estabilidad del punto fijo.

En esta charla, veremos que la estabilidad KAM por sí sola no es suficiente para garantizar la estabilidad en el sentido de Lyapunov para puntos fijos elípticos de hamiltonianos suaves con dos grados de libertad. Construiremos tales ejemplos para cualquier vector de frecuencia con coordenadas de distinto signo.

Dia: Dimecres, 29 de març de 2023

Lloc: Aula S04, Facultat de Matemàtiques i Estadística, UPC. Pau Gargallo,14 BCN.

- Hora: 16h00m

A càrrec de: Robert Cardona (UPM-ICMAT)

Títol: Periodic orbits and surfaces of section on Hamiltonian systems: from contact to stable energy level sets

Resum: The quest for periodic orbits and surfaces of section (or "Birkhoff sections") traces back to the work of Poincaré in his study of the restricted three-body problem. Proving the existence of these two objects on regular energy level sets of Hamiltonian systems has been a leading problem in the flourishing field of symplectic dynamics. In this talk, we are interested in the dynamics of non-vanishing volume-preserving vector fields on closed three-manifolds, equivalently understood as Hamiltonian vector fields along regular energy level sets. We will survey some recent striking results for the class of Reeb vector fields defined by contact forms, and present generalizations to the case of vector fields defined by stable Hamiltonian structures. Such structures appear, for example, when restricting a Hamiltonian vector field to a "stable" energy level set.

This talk is based on joint work with A. Rechtman.

Dia: Dimecres, 12 d'abril de 2023

Lloc: Aula T2 (segon pis), Facultat de Matemàtiques i Informàtica, UB.

- Hora: 16h00m

A càrrec de: Santiago Barbieri (Université Paris-Saclay and Università degli Studi Roma Tre)

Títol: Analytic smoothing and Nekhoroshev estimates for Hölder steep Hamiltonians

Resum: In this seminar, I will present the first result of Nekhoroshev
stability for steep Hamiltonians in Hölder class. Our new approach combines
the classical theory of normal forms in analytic category with an improved
smoothing procedure to approximate an Hölder Hamiltonian with an analytic
one. It is only for the sake of clarity that we consider the case of Hölder
perturbations of an analytic integrable Hamiltonian, but our method is
flexible enough to work in many other functional classes, including the
Gevrey one. The stability exponents can be taken to be
(l-1)/(2n α_{1} ... α_{n-2}) + 1/2 for the time of stability and
1/(2n α_{1} ... α_{n-2}) for the radius of stability, n being the
dimension, l>n+1 being the regularity and the α_{i}’s being the indices
of steepness. Crucial to obtain the exponents above is a new non-standard
estimate on the Fourier norm of the smoothed function. As a byproduct we
improve the stability exponents in the 𝒞^{k} class, with integer k.

Joint work with J.P. Marco and J. Massetti.

Dia: Dimecres, 19 d'abril de 2023

Lloc: Aula S04, Facultat de Matemàtiques i Estadística, UPC. Pau Gargallo,14 BCN.

- Hora: 16h00m

A càrrec de: Gian Marco Canneori (Università di Torino)

Títol: The N-centre problem on Riemannian surfaces: a variational approach.

Resum: The classical N-centre problem of Celestial Mechanics describes the behaviour of a point particle under the attraction of a finite number of motionless bodies. Considered as a limit case of a (N+1)-body problem, it has been the object of several results concerning integrability, investigation of chaos and existence of periodic orbits, mostly when the motion is constrained to the Euclidean plane. In particular, variational approaches are convincing in this situation and have produced classes of collision-less periodic solutions, after imposing topological constraints of different natures. Looking for genuine solutions of second order differential equations, the most delicate step resides in avoiding collisions with the centres. Picturing a more realistic situation, a natural extension of these results could be the one in which the motion is constrained to a prescribed Riemannian surface. In this talk we state the N-centre problem on orientable surfaces and we show how it is possible to use variational arguments in order to produce a huge set of collision-less periodic solutions. Such trajectories will be found among homotopy classes of loops, and their variational and topological properties will be described. Also, the major difficulties and differences with the planar case will be addressed.

This is a joint work with Stefano Baranzini from the University of Torino.

Sessió actual.

Last updated: Monday, 24-Apr-2023 12:02:31 CEST