Quantifying lawlessness in finitely generated groups

Abstract

We introduce a quantitative notion of lawlessness for finitely generated groups, encoded by the "lawlessness growth function" $\mathcal{A}_{\Gamma} : \mathbb{N} \rightarrow \mathbb{N}$. We show that $\mathcal{A}_{\Gamma}$ is bounded iff $\Gamma$ has a nonabelian free subgroup. By contrast we construct, for any nondecreasing unbounded function $f: \mathbb{N} \rightarrow \mathbb{N}$, an elementary amenable lawless groups for which $\mathcal{A}_{\Gamma}$ grows more slowly that $f$. We produce torsion lawless groups for which $\mathcal{A}_{\Gamma}$ is at least linear using Golod-Shafarevich theory, and give some upper bounds on $\mathcal{A}_{\Gamma}$ for Grigorchuk's group and Thompson's group $\mathbf{F}$. We note some connections between $\mathcal{A}_{\Gamma}$ and quantitative versions of residual finiteness. Finally, we also describe a function $\mathcal{M}_{\Gamma}$ quantifying the property of $\Gamma$ having no mixed identities, and give bounds for nonabelian free groups. By contrast with $\mathcal{A}_{\Gamma}$, there are no groups for which $\mathcal{M}_{\Gamma}$ is bounded: we prove a universal lower bound on $\mathcal{M}_{\Gamma}(n)$ of the order of $\log (n)$.