On the self-similarity index of $p$-adic analytic pro-$p$ groups

Abstract

Let $p$ be a prime. We say that a pro-$p$ group is self-similar of index $p^k$ if it admits a faithful self-similar action on a $p^k$-ary regular rooted tree such that the action is transitive on the first level. The self-similarity index of a self-similar pro-$p$ group $G$ is defined to be the least power of $p$, say $p^k$, such that $G$ is self-similar of index $p^k$. We show that for every prime $p⩾3$ and all integers $d$ there exist infinitely many pairwise non-isomorphic self-similar 3-dimensional hereditarily just-infinite uniform pro-$p$ groups of self-similarity index greater than $d$. This implies that, in general, for self-similar $p$-adic analytic pro-$p$ groups one cannot bound the self-similarity index by a function that depends only on the dimension of the group.

9 pages