Mon, 20 Jan 2025
16:30
L4

Fluctuations around the mean-field limit for attractive Riesz interaction kernels in the moderate regime

Alexandra Holzinger
(Mathematical Institute)
Abstract

In this talk I will give a short introduction to moderately interacting particle systems and the general notion of fluctuations around the mean-field limit. We will see how a central limit theorem can be shown for moderately interacting particles on the whole space for certain types of interaction potentials. The interaction potential approximates singular attractive potentials of sub-Coulomb type and we can show that the fluctuations become asymptotically Gaussians. The methodology is inspired by the classical work of Oelschläger in the 1980s on fluctuations for the porous-medium equation. To allow for attractive potentials we use a new approach of quantitative mean-field convergence in probability in order to include aggregation effects. 

Mon, 27 Jan 2025
16:30
L4

Sampling with Minimal Energy

Ed Saff
(Vanderbilt University)
Abstract

Minimal discrete energy problems arise in a variety of scientific contexts – such as crystallography, nanotechnology, information theory, and viral morphology, to name but a few.     Our goal is to analyze the structure of configurations generated by optimal (and near optimal)-point configurations that minimize the Riesz s-energy over a sphere in Euclidean space R^d and, more generally, over a bounded manifold. The Riesz s-energy potential, which is a generalization of the Coulomb potential, is simply given by 1/r^s, where r denotes the distance between pairs of points. We show how such potentials for s>d and their minimizing point configurations are ideal for use in sampling surfaces.

Connections to the results by Field's medalist M. Viazovska and her collaborators on best-packing and universal optimality in 8 and 24 dimensions will be discussed. Finally we analyze the minimization of a "k-nearest neighbor" truncated version of Riesz energy that reduces the order N^2 computation for energy minimization to order N log N , while preserving global and local properties.

Sat, 20 Jan 2024
16:30
L4

TBC

Noureddine Igbida
(Université de Limoges)
Acoustic interaction of a finite body in a rarefied gas: does sound reciprocity hold at non-continuum conditions?
Manela, A Ben-Ami, Y Journal of Fluid Mechanics volume 999 a105 (25 Nov 2024)
Prevalence of ρ-irregularity and related properties
Galeati, L Gubinelli, M Annales de l Institut Henri Poincaré Probabilités et Statistiques volume 60 issue 4 (01 Nov 2024)
Thu, 12 Dec 2024
14:00
(This talk is hosted by Rutherford Appleton Laboratory)

A Subspace-conjugate Gradient Method for Linear Matrix Equations

Davide Palitta
(Università di Bologna)
Abstract

 
The solution of multiterm linear matrix equations is still a very challenging task in numerical linear algebra.
If many different solid methods for equations with (at most) two terms exist in the literature, having a number of terms greater than two makes the numerical treatment of these equations much trickier. Very few options are available in the literature. In particular, to the best of our knowledge, no decomposition-based method for multiterm equations has never been proposed; only iterative procedures exist.
A non-complete list of contributions in this direction includes a greedy procedure designed by Sirkovi\'c and Kressner, projection methods tailored to the equation at hand, Riemannian optimization schemes, and matrix-oriented Krylov methods with low-rank truncations. The last class of solvers is probably one of the most commonly used ones. These schemes amount to adapting standard Krylov schemes for linear systems to matrix equations by leveraging the equivalence between matrix equations and their Kronecker form.
As long as no truncations are performed, this equivalence means that the algorithm itself is not exploiting the structure of the problem as it is not able to see that we are actually solving a matrix equation and not a linear system. The low-rank truncations we may perform in the matrix-equation framework can be viewed as a simple computational tool needed to make the solution process affordable in terms of storage allocation and not as an algorithmic advance.

 
By taking inspiration from the matrix-oriented cg method, our goal is to design a novel iterative scheme for the solution of multiterm matrix equations. We name this procedure the subspace-conjugate gradient method (Ss-cg) due to the peculiar orthogonality conditions imposed to compute some of the quantities involved in the scheme. As we will show in this talk, the main difference between Ss-cg and cg is the ability of the former to capitalize on the matrix equation structure of the underlying problem. In particular, we will make full use of the (low-rank) matrix format of the iterates to define appropriate ``step-sizes''. If these quantities correspond to scalars alpha_k and beta_k in cg, they will amount to small-dimensional matrices in our fresh approach.
This novel point of view leads to remarkable computational gains making Ss-cg a very competitive option for the solution of multi-term linear matrix equations.

 
This is a joint work with Martina Iannacito and Valeria Simoncini, both from the Department of Mathematics, University of Bologna.
 
Mon, 02 Dec 2024
13:30
C4

Extended TQFT, gauge theory, and Measurement Based Quantum Computation

Gabriel Wong
Abstract

Measurement-Based Quantum Computation (MBQC) is a model of quantum computation driven by measurements instead of unitary gates.   In 2D it is capable of supporting universal quantum computations.   Interestingly, while all measurements are local, the computational output involves non local observables.   We will use the simpler case of 1D MBQC to illustrate how these features can be captured by ideas from gauge theory and extended TQFT. We will also explain  MBQC from the perspective of the extended Hilbert space construction in gauge theories, in which the entanglement edge modes play the role of the logical qubit.

Eighty-six billion and counting: do we know the number of neurons in the human brain?
Goriely, A Brain : a journal of neurology volume 148 issue 3 689-691 (Mar 2025)
Thu, 27 Feb 2025

16:00 - 17:00
Lecture Room 4

The wild Brauer-Manin obstruction

Margherita Pagano
(Imperial College London)
Abstract

A way to study rational points on a variety is by looking at their image in the p-adic points. Some natural questions that arise are the following: is there any obstruction to weak approximation on the variety? Which primes might be involved in it? I will explain how primes of good reduction can play a role in the Brauer-Manin obstruction to weak approximation, with particular emphasis on the case of K3 surfaces.

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