On arithmetic properties of solvable Baumslag-Solitar groups

TL;DR

This paper studies the geometric growth properties of arithmetic box spaces of solvable Baumslag-Solitar groups, establishing bounds on their diameters relative to size and exploring a version of the congruence subgroup property.

Contribution

It introduces new bounds on property D_α for these box spaces and proves a CSP-like property for subgroups of certain matrix groups.

Findings

Arithmetic box spaces have property D_α only if α ≤ 1/2.

Finitely supported prime levels yield D_{1/2} property.

Supports on primes with positive density do not have D_α for any α > 0.

Abstract

For $0<\alpha\le 1$, we say that a sequence $(X_k)_{k>0}$ of $d$-regular graphs has property $D_\alpha$ if there exists a constant $C>0$ such that $\mathrm{diam}(X_k)\ge C\cdot|X_k|^\alpha$. We investigate property $D_\alpha$ for arithmetic box spaces of the solvable Baumslag-Solitar groups $BS(1,m)$ (with $m\geq 2$): those are box spaces obtained by embedding $BS(1,m)$ into the upper triangular matrices in $GL_2(\mathbb{Z}[1/m])$ and intersecting with a family $M_{N_k}$ of congruence subgroups of $GL_2(\mathbb{Z}[1/m])$, where the levels $N_k$ are coprime with $m$ and $N_k|N_{k+1}$. We prove: - if an arithmetic box space has $D_\alpha$, then $\alpha\le\frac{1}{2}$~; - if the family $(N_k)_k$ of levels is supported on finitely many primes, the corresponding arithmetic box space has $D_{1/2}$~; - if the family $(N_k)_k$ of levels is supported on a family of primes with positive analytic primitive density, then the corresponding arithmetic box space does not have $D_\alpha$, for every $\alpha>0$. Moreover, we prove that if we embed $BS(1,m)$ in the group of invertible upper-triangular matrices $T_n(\mathbb{Z}[1/m])$, then every finite index subgroup of the embedding contains a congruence subgroup. This is a version of the congruence subgroup property (CSP).