Distality of Certain Actions on $p$-adic Spheres

Abstract

Consider the action of $GL(n,\mathbb{Q_p})$ on the $p$-adic unit sphere $\mathcal{S}_n$ arising from the linear action on $\mathbb{Q}_p^n\setminus\{0\}$. We show that for the action of a semigroup $\mathfrak{S}$ of $GL(n,\mathbb{Q}_p)$ on $\mathcal{S}_n$, the following are equivalent: (1) $\mathfrak{S}$ acts distally on $\mathcal{S}_n$. (2) the closure of the image of $\mathfrak{S}$ in $PGL(n,\mathbb{Q}_p)$ is a compact group. On $\mathcal{S}_n$, we consider the `affine' maps $\overline{T}_a$ corresponding to $T$ in $GL(n,\mathbb{Q}_p)$ and a nonzero $a$ in $\mathbb{Q}_p^n$ satisfying $\|T^{-1}(a)\|_p<1$. We show that there exists a compact open subgroup $V$, which depends on $T$, such that $\overline{T}_a$ is distal for every nonzero $a\in V$ if and only if $T$ acts distally on $\mathcal{S}_n$. The dynamics of `affine' maps on $p$-adic unit spheres is quite different from that on the real unit spheres.