Asymptotic Homotopical Complexity of an Infinite Sequence of Dispersing $2D$ Billiards

Abstract

We investigate the large scale chaotic, topological structure of the trajectories of an infinite sequence of dispersing, hence ergodic, $2D$ billiards with the configuration space $Q_n=\mathbb{T}^2 \setminus \bigcup_{i=0}^{n-1} D_i$, where the scatterers $D_i$ ($i=0,1,\dots,n-1$) are disks of radius $r<<1$ centered at the points $(i/n, 0)$ mod $\mathbb{Z}^2$. We get effective lower and upper radial bounds for the rotation set $R$. Furthermore, we also prove the compactness of the admissible rotation set $AR$ and the fact that the rotation vectors $v$ corresponding to admissible periodic orbits form a dense subset of $AR$. We also obtain asymptotic lower and upper estimates for the sequence $h_{top}(n)$ of topological entropies and precise asymptotic formulas for the metric entropies $h_{\mu}(n,r)$.