Degrees of compression and inertia for free-abelian times free groups

Abstract

We introduce the concepts of degree of inertia, ${\text{di}}_G(H)$, and degree of compression, ${\text{dc}}_G(H)$, of a finitely generated subgroup $H$ of a given group $G$. For the case of direct products of free-abelian and free groups, we compute the degree of compression and give an upper bound for the degree of inertia. Imposing some technical assumptions to the supremum involved in the definition of degree of inertia, we introduce the notion called restricted degree of inertia, ${\text{di}}_G^{\prime }(H)$, and, again for the case ${\mathbb{Z}}^m\times F_n$, we provide an explicit formula relating it to the restricted degree of inertia of its projection to the free part, ${\text{di}}_{F_n}^{\prime }(H\pi )$.