Subgroup measures and profinite rigidity in free groups

Abstract

Let $H$ be a finitely generated subgroup of a finitely generated group $F$. Restriction of a uniformly random homomorphism $F\to G$, with $G$ finite, defines a probability measure on $\mathrm{Hom}(H,G)$; for cyclic $H$ this is the usual word measure. We introduce marked and setwise notions of profinite rigidity for these subgroup measures. In a finitely generated LERF group, an isomorphism between finitely generated subgroups preserves all induced measures if and only if its profinite completion extends to an automorphism of the ambient profinite completion. This gives a discrete - profinite orbit criterion and identifies the precise obstruction separating marked from setwise rigidity. For free groups, both notions are invariant under adjoining a free factor. We classify the measure classes of subgroups of $F_n$ containing $[F_n,F_n]$: they are the $\mathrm{Aut}(F_n)$-orbits determined by Smith invariants, and the subgroups determined uniquely by their measures are exactly $F_n^m[F_n,F_n]$. Finally, marked rigidity is preserved and reflected by free products compatible with an ambient free-product decomposition. Consequently, for every $n\ge 2$ and $1\le r\le n$, there is an infinite-index profinitely rigid subgroup of $F_n$ of rank $r$ whose algebraic closure is $F_n$.

26 pages