Splitting schemes are a natural and easy to implement approach to integrate numerically in time differential equations. However, high order splitting methods suffer in general from an order reduction phenomena when applied to the integration of partial differential equations with non-periodic boundary conditions. In this talk, inspired by recent corrector techniques for the second order Strang splitting method, we present a new splitting method of order three for a class of semilinear parabolic problems that avoids order reduction. We prove the third order convergence of the method in a simplified linear setting and confirm the result by numerical experiments. Moreover, we observe numerically that the high order convergence persists for an order four variant of a splitting method, and also for a nonlinear source term.

This presentation focuses on the Vlasov-Poisson system with and without collisions. This kinetic model encodes the multiple scales that arise in a plasma, ranging from fluid-like behavior when collisions dominate, to wave interactions in weakly collisional regimes. We present a numerical method for the Vlasov-Poisson system which preserves its structure in both collision and collisionless regimes. We explain the key ideas in order to preserve the energy structure of the system and its large time behavior in collisional settings. We also show that the method adapts to higher dimensional frameworks.

A differential graded algebraic structure A (e.g. an associative algebra, a Lie algebra, an operad, etc.) is formal if it is related to its homology H(A) by a zig-zag of quasi-isomorphisms preserving the algebraic structure. Kaledin classes were introduced as an obstruction theory fully characterizing the formality of associative algebras over a characteristic zero field. In this talk, I will present a generalization of Kaledin classes to any coefficients ring, to other algebraic structures (encoded by operads, possibly colored, or by properads), and to address a more general problem: the existence of homotopy equivalences between algebraic structures. I will prove new formality and homotopy equivalence results based on this obstruction theory, presenting applications in several domains such as algebraic geometry, representation theory and mathematical physics.

In this overview talk, I will present the encounter-based approach to diffusive processes in Euclidean domains and highlight its fundamental relation to the Steklov spectral problem. So, the Steklov eigenfunctions turn out to be particularly useful for representing heat kernels with Robin boundary condition and describing diffusive dynamics with reaction events on the boundary. In the second part of the talk, I will discuss the asymptotic behavior of the Steklov eigenvalues for the exterior Steklov problem. Some open questions related to spectral, probabilistic and asymptotic aspects of this problem will be outlined.

References:

[1] D. S. Grebenkov, Paradigm Shift in Diffusion-Mediated Surface Phenomena, Phys. Rev. Lett. 125, 078102 (2020).

[2] D. S. Grebenkov and A. Chaigneau, The Steklov problem for exterior domains: asymptotic behavior and applications (accepted to J. Math. Phys.; preprint ArXiv 2407.09864v2)

Two smoothly embedded surfaces in a smooth 4-manifold are called exotic if they are topologically isotopic but not smoothly isotopic. The phenomenon of exotic surfaces in dimension 4 is an interesting topic in low-dimensional topology. In this talk, we start with an introduction to equivariant knot concordance theory in the context of exotic disks. Then we recall how involutive Heegaard Floer theory can work as a machinery for equivariant concordance. Then we demonstrate our recent progress on equivariant concordance of Whitehead doubles, which produces exotic disk pairs.

This is joint work with Sungkyung Kang and JungHwan Park.

In this short lecture, I will explain the definition of Lefschetz fibrations on 4-manifolds and basic properties of them. In particular, I will describe a classification theorem of Kas and Matsumoto which asserts that there is one to one correspondence between the isomorphism classes of Lefschetz fibrations and the conjugacy classes of monodromy

On s'intéresse aux marches aléatoires en milieu aléatoire. La difficulté principale dans l'étude de ces systèmes est l'interaction très forte entre la marche et son environnement. Dans cet exposé, on propose un critère qui, lorsqu'il est satisfait, permet de décomposer la trajectoire de la marche en incréments iid, et à terme de démontrer des théorèmes limites. Le critère porte sur l'environnement et implique la construction d'un champ aléatoire qui satisfait une certaine propriété de Markov ainsi que des estimées de décorrélation. On applique ce critère à des environnements corrélés en temps comme la percolation booléenne sur Z^d x N et des chaînes de renouvellement iid en espace. Basé sur https://arxiv.org/abs/2409.12515 (travail réalisé avec J. Allasia, R. Baldasso et A. Teixeira).