p-adic limits of renormalized logarithmic Euler characteristics

TL;DR

This paper investigates p-adic limits of Euler characteristics for residually finite groups, establishing conditions under which these limits exist and relate to p-adic torsion, extending entropy concepts into the p-adic setting.

Contribution

It introduces p-adic expansiveness conditions ensuring well-defined Euler characteristic limits and links these limits to p-adic R-torsion, generalizing entropy notions in the p-adic context.

Findings

Established conditions for the existence of p-adic limits of Euler characteristics.

Proved the limit equals the p-adic R-torsion under certain conditions.

Connected the theory to Serre's intersection numbers on arithmetic schemes.

Abstract

Given a countable residually finite group $\Gamma$, we write $\Gamma_n \to e$ if $(\Gamma_n)$ is a sequence of normal subgroups of finite index such that any infinite intersection of $\Gamma_n$'s contains only the unit element $e$ of $\Gamma$. Given a $\Gamma$-module $M$ we are interested in the multiplicative Euler characteristics \begin{equation} \chi (\Gamma_n , M) = \prod_i |H_i (\Gamma_n , M)|^{(-1)^i} \end{equation} and the limit in the field $\mathbb{Q}_p$ of $p$-adic numbers \begin{equation} h_p := \lim_{n\to\infty} (\Gamma : \Gamma_n)^{-1} \log_p \chi (\Gamma_n , M) \; . \end{equation} Here $\log_p : \mathbb{Q}^{\times}_p \to \mathbb{Z}_p$ is the branch of the $p$-adic logarithm with $\log_p (p) = 0$. Of course, neither expression will exist in general. We isolate conditions on $M$, in particular $p$-adic expansiveness which guarantee that the Euler characteristics $\chi (\Gamma_n , M)$ are well defined. That notion is a $p$-adic analogue of expansiveness of the dynamical system given by the $\Gamma$-action on the compact Pontrjagin dual $X = M^*$ of $M$. Under further conditions on $\Gamma$ we also show that the renormalized $p$-adic limit in the second formula exists and equals the $p$-adic $R$-torsion of $M$. The latter is a $p$-adic analogue of the Li--Thom $L^2$ $R$-torsion of a $\Gamma$-module $M$ which they related to the entropy $h$ of the $\Gamma$-action on $X$. We view the limit $h_p$ as a version of entropy which values in the $p$-adic numbers and the equality with $p$-adic $R$-torsion as an analogue of the Li--Thom formula in the expansive case. We discuss the case $\Gamma = \mathbb{Z}^N$ in more detail where our theory is related to Serre's intersection numbers on arithmetic schemes.