Commuting Probability of Compact Groups

Abstract

For any (Hausdorff) compact group $G$ with the normalized Haar measure ${\mathbf{m}}_G$, denote by $\mathrm{cp}(G)$ the probability ${\mathbf{m}}_{G\times G}(\{(x,y)\in G\times G|xy=yx\})$ of commuting a randomly chosen pair of elements of $G$. Here we prove that if $\mathrm{cp}(G)>0$, then there exists a finite group $H$ such that $\mathrm{cp}(G)=\frac{\mathrm{cp}(H)}{|G:F{|}^2}$, where $F$ is the FC-center of $G$ i.e. the set of all elements of $G$ whose conjugacy classes are finite and $H$ is isoclinic to $F$ with $\mathrm{cp}(F)=\mathrm{cp}(H)$. The latter equality enables one to transfer many existing results concerning commuting probability of finite groups to one of compact groups. For example, here for a compact group $G$ we prove that if $\mathrm{cp}(G)>\frac{3}{40}$ then either $G$ is solvable or, else $G\cong A_5\times T$ for some abelian group $T$, in which case $\mathrm{cp}(G)=\frac{1}{12}$; where $A_5$ denotes the alternating group of degree $5$.