Sharp Asymptotics for Abelian Covers of Groups with Bounded Noncommutativity

Abstract

We determine the sharp exponential growth rate of the minimum number of abelian subgroups required to cover a group with bounded pairwise noncommutativity. Let $\omega (G)$ denote the largest size of a pairwise noncommuting subset of a group $G$, let $a(G)$ be the least size of an abelian cover, and define $h(n)=\sup \{a(G):\omega (G)\le n\}$. We prove the quantitative estimate ${\log }_{2}h(n)=n/2+O(\sqrt{n}(\log (n+2){)}^3)$, and hence $h(n{)}^{1/n}\to \sqrt{2}$. Extraspecial $2$-groups give the matching lower bound. For the upper bound, we reduce to finite groups by isoclinism, analyze finite $p$-groups through a central series of the derived subgroup and alternating commutator forms, control interactions between central factors, and then pass through Sylow decomposition and the Fitting subgroup at polynomial cost. The argument also determines the same sharp exponential rate for the least possible index of an abelian subgroup and shows that asymptotic extremality is concentrated in $2$-groups. This result resolves Erdős Problem #117 at the level of its sharp exponential asymptotics.

19 pages. Submitted to Graphs and Combinatorics