Lower bounds on the Lyapunov exponents of stochastic differential equations

TL;DR

This paper reviews new methods for establishing positive lower bounds on the top Lyapunov exponent in high-dimensional stochastic differential equations, combining identities with hypoelliptic regularity estimates to prove chaos in models like Lorenz-96 and 2D Navier-Stokes.

Contribution

It introduces a novel approach linking Lyapunov exponents to Fisher information and hypoelliptic regularity, providing the first rigorous bounds for certain stochastic PDE models.

Findings

Established positive lower bounds for Lyapunov exponents in Lorenz-96 and Galerkin Navier-Stokes models.

Connected Lyapunov exponents to Fisher information of stationary measures.

Verified Hörmander's condition for the projective process in specific stochastic PDEs.

Abstract

In this article, we review our recently introduced methods for obtaining strictly positive lower bounds on the top Lyapunov exponent of high-dimensional, stochastic differential equations such as the weakly-damped Lorenz-96 (L96) model or Galerkin truncations of the 2d Navier-Stokes equations. This hallmark of chaos has long been observed in these models, however, no mathematical proof had been made for either deterministic or stochastic forcing. The method we proposed combines (A) a new identity connecting the Lyapunov exponents to a Fisher information of the stationary measure of the Markov process tracking tangent directions (the so-called "projective process"); and (B) an $L^1$-based hypoelliptic regularity estimate to show that this (degenerate) Fisher information is an upper bound on some fractional regularity. For L96 and GNSE, we then further reduce the lower bound of the top Lyapunov exponent to proving that the projective process satisfies H\"ormander's condition. We review the recent contributions of the first and third author on the verification of this condition for the 2d Galerkin-Navier-Stokes equations in a rectangular, periodic box of any aspect ratio. Finally, we briefly contrast this work with our earlier work on Lagrangian chaos in the stochastic Navier-Stokes equations. We end the review with a discussion of some open problems.