Thermodynamic formalism for dispersing billiards

For any finite horizon Sinai billiard map \begin{document}$T$\end{document} on the two-torus, we find \begin{document}$t_*>1$\end{document} such that for each \begin{document}$t\in (0,t_*)$\end{document} there exists a unique equilibrium state \begin{document}$\mu_t$\end{document} for \begin{document}$- t\log J^uT$\end{document}, and \begin{document}$\mu_t$\end{document} is \begin{document}$T$\end{document}-adapted. (In particular, the SRB measure is the unique equilibrium state for \begin{document}$- \log J^uT$\end{document}.) We show that \begin{document}$\mu_t$\end{document} is exponentially mixing for Hölder observables, and the pressure function \begin{document}$P(t) = \sup_\mu \{h_\mu -\int t\log J^uT d \mu\}$\end{document} is analytic on \begin{document}$(0,t_*)$\end{document}. In addition, \begin{document}$P(t)$\end{document} is strictly convex if and only if \begin{document}$\log J^uT$\end{document} is not \begin{document}$\mu_t$\end{document}-a.e. cohomologous to a constant, while, if there exist \begin{document}$t_a\ne t_b$\end{document} with \begin{document}$\mu_{t_a} = \mu_{t_b}$\end{document}, then \begin{document}$P(t)$\end{document} is affine on \begin{document}$(0,t_*)$\end{document}. An additional sparse recurrence condition gives \begin{document}$\lim_{t\downarrow 0} P(t) = P(0)$\end{document}.

Authors:
;
Award ID(s):
Publication Date:
NSF-PAR ID:
10382743
Journal Name:
Journal of Modern Dynamics
Volume:
18
Issue:
0
Page Range or eLocation-ID:
415
ISSN:
1930-5311
1. Consider the linear transport equation in 1D under an external confining potential \begin{document}$\Phi$\end{document}:
For \begin{document}$\Phi = \frac {x^2}2 + \frac { \varepsilon x^4}2$\end{document} (with \begin{document}$\varepsilon >0$\end{document} small), we prove phase mixing and quantitative decay estimates for \begin{document}${\partial}_t \varphi : = - \Delta^{-1} \int_{ \mathbb{R}} {\partial}_t f \, \mathrm{d} v$\end{document}, with an inverse polynomial decay rate \begin{document}$O({\langle} t{\rangle}^{-2})$\end{document}. In the proof, we develop a commuting vector field approach, suitably adapted to this setting. We will explain why we hope this is relevant for the nonlinear stability of the zero solution for the Vlasov–Poisson system in \begin{document}$1$\end{document}D under the external potential \begin{document}$\Phi$\end{document}.
2. This paper studies a family of generalized surface quasi-geostrophic (SQG) equations for an active scalar \begin{document}$\theta$\end{document} on the whole plane whose velocities have been mildly regularized, for instance, logarithmically. The well-posedness of these regularized models in borderline Sobolev regularity have previously been studied by D. Chae and J. Wu when the velocity \begin{document}$u$\end{document} is of lower singularity, i.e., \begin{document}$u = -\nabla^{\perp} \Lambda^{ \beta-2}p( \Lambda) \theta$\end{document}, where \begin{document}$p$\end{document} is a logarithmic smoothing operator and \begin{document}$\beta \in [0, 1]$\end{document}. We complete this study by considering the more singular regime \begin{document}$\beta\in(1, 2)$\end{document}. The main tool is the identification of a suitable linearized system that preserves the underlying commutator structure for the original equation. We observe that this structure is ultimately crucial for obtaining continuity of the flow map. In particular, straightforward applications of previous methods for active transport equations fail to capture the more nuanced commutator structure of the equation in this more singular regime. The proposed linearized system nontrivially modifies the flux of the original system in such a way that it coincides with the original flux when evaluated along solutions of themore »
3. Stochastic differential games have been used extensively to model agents' competitions in finance, for instance, in P2P lending platforms from the Fintech industry, the banking system for systemic risk, and insurance markets. The recently proposed machine learning algorithm, deep fictitious play, provides a novel and efficient tool for finding Markovian Nash equilibrium of large \begin{document}$N$\end{document}-player asymmetric stochastic differential games [J. Han and R. Hu, Mathematical and Scientific Machine Learning Conference, pages 221-245, PMLR, 2020]. By incorporating the idea of fictitious play, the algorithm decouples the game into \begin{document}$N$\end{document} sub-optimization problems, and identifies each player's optimal strategy with the deep backward stochastic differential equation (BSDE) method parallelly and repeatedly. In this paper, we prove the convergence of deep fictitious play (DFP) to the true Nash equilibrium. We can also show that the strategy based on DFP forms an \begin{document}$\epsilon$\end{document}-Nash equilibrium. We generalize the algorithm by proposing a new approach to decouple the games, and present numerical results of large population games showing the empirical convergence of the algorithm beyond the technical assumptions in the theorems.
4. We establish an instantaneous smoothing property for decaying solutions on the half-line \begin{document}$(0, +\infty)$\end{document} of certain degenerate Hilbert space-valued evolution equations arising in kinetic theory, including in particular the steady Boltzmann equation. Our results answer the two main open problems posed by Pogan and Zumbrun in their treatment of \begin{document}$H^1$\end{document} stable manifolds of such equations, showing that \begin{document}$L^2_{loc}$\end{document} solutions that remain sufficiently small in \begin{document}$L^\infty$\end{document} (i) decay exponentially, and (ii) are \begin{document}$C^\infty$\end{document} for \begin{document}$t>0$\end{document}, hence lie eventually in the \begin{document}$H^1$\end{document} stable manifold constructed by Pogan and Zumbrun.
5. We study the convergence rate of a continuous-time simulated annealing process \begin{document}$(X_t; \, t \ge 0)$\end{document} for approximating the global optimum of a given function \begin{document}$f$\end{document}. We prove that the tail probability \begin{document}$\mathbb{P}(f(X_t) > \min f +\delta)$\end{document} decays polynomial in time with an appropriately chosen cooling schedule of temperature, and provide an explicit convergence rate through a non-asymptotic bound. Our argument applies recent development of the Eyring-Kramers law on functional inequalities for the Gibbs measure at low temperatures.