Intersections and joins of free groups

TL;DR

This paper establishes a strong inequality relating the ranks of intersections and joins of subgroups in free groups, with implications for hyperbolic 3-manifold volumes and a special case of the Hanna Neumann Conjecture.

Contribution

It proves a new strong form of Burns' inequality and a particular case of the Hanna Neumann Conjecture for free groups.

Findings

Proved a strong inequality for subgroup intersections and joins.

Derived a corollary used in hyperbolic 3-manifold volume analysis.

Established a specific case of the Hanna Neumann Conjecture.

Abstract

Let H and K be subgroups of a free group of ranks h and k \geq h. We prove the following strong form of Burns' inequality: rank(H \cap K) - 1 \leq 2(h-1)(k-1) - (h-1)(rank(H \vee K) -1). A corollary of this, also obtained by L. Louder and D. B. McReynolds, has been used by M. Culler and P. Shalen to obtain information regarding the volumes of hyperbolic 3-manifolds. We also prove the following particular case of the Hanna Neumann Conjecture, which has also been obtained by Louder. If the join of H and K has rank at least (h + k + 1)/2, then the intersection of H and K has rank no more than (h-1)(k-1) + 1.