Wed, 03 Dec 2025

12:00 - 13:00
C1

AOT algorithm for a system of equations arising in meteorology

Piotr Gwiazda
(Institute of Mathematics of the Polish Academy of Sciences, Interdisciplinary Centre for Mathematical and Computational Modelling of University of Warsaw)
Abstract

Data assimilation plays a crucial role in modern weather prediction, providing a systematic way to incorporate observational data into complex dynamical models. The paper addresses continuous data assimilation for a model arising as a singular limit of the three-dimensional compressible Navier-Stokes-Fourier system with rotation driven by temperature gradient. The limit system preserves the essential physical mechanisms of the original model,  while exhibiting a reduced, effectively two-and-a-half-dimensional structure. This simplified framework allows for a rigorous analytical study of the data assimilation process while maintaining a direct physical connection to the full compressible model.  We establish well posedness of global-in-time solutions and a compact trajectory attractor, followed by the stability and convergence results for the nudging scheme applied to the limiting system. Finally, we demonstrate how these results can be combined with a relative entropy argument to extend the assimilation framework to the full three-dimensional compressible setting, thereby establishing a rigorous connection between the reduced and physically complete models.

Thu, 04 Dec 2025
11:00
C6

Paradoxical decompositions and spectral gaps for linear group actions

Emmanuel Breuillard
(Oxford University)
Abstract
By a celebrated theorem of Tarski, a (discrete) group action has no finitely additive invariant measure  (i.e. is "non-amenable") if and only if it exhibits a paradoxical decomposition, and if and only if it admits a spectral gap. We prove yet another equivalence by introducing the notion of "ping-pong with overlaps", which we then apply to characterise non-amenable algebraic actions of linear groups over any field and show that they are uniformly non-amenable uniformly over all fields. The proof makes key use of diophantine heights. Joint work with Oren Becker.
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