An estimate for the entropy of Hamiltonian flows

Abstract

In the paper we present a generalization to Hamiltonian flows on symplectic manifolds of the estimate proved by Ballmann and Wojtkovski in \cite{BaWoEnGeo} for the dynamical entropy of the geodesic flow on a compact Riemannian manifold of nonpositive sectional curvature. Given such a Riemannian manifold $M,$ Ballmann and Wojtkovski proved that the dynamical entropy $h_{\mu }$ of the geodesic flow on $M$ satisfies the following inequality: $$ h_μ \geq \int_{SM} \traccia \sqrt{-K(v)} dμ(v), $$ \noindent where $v$ is a unit vector in $T_{p}M$, if $p$ is a point in $M$, $SM$ is the unit tangent bundle on $M,$ $K(v)$ is defined as $K(v)=\mathcal{R}(\cdot ,v)v$, with $\mathcal{R}$ Riemannian curvature of $M$, and $\mu$ is the normalized Liouville measure on $SM$. We consider a symplectic manifold $M$ of dimension $2n$, and a compact submanifold $N$ of $M,$ given by the regular level set of a Hamiltonian function on $M$; moreover we consider a smooth Lagrangian distribution of rank $n-1$ on $N,$ and we assume that the reduced curvature ${\hat{R}}_z^h$ of the Hamiltonian vector field $\vec{h}$ is nonpositive. Then we prove that under these assumptions the dynamical entropy $h_{\mu }$ of the Hamiltonian flow w.r.t. the normalized Liouville measure on $N$ satisfies: h_μ \geq \int_N \traccia \sqrt{-\hat{R}_z^h} dμ.

10 pages