On the intersection of free subgroups in free products of groups with no 2-torsion

Warren Dicks, S. V. Ivanov

Research output: Contribution to journalArticlepeer-review


Let (GℓL) be a family of groups and let F be a free group. Let G denote FℓLG, the free product of F and all the G. Let denote the set of all finitely generated (free) subgroups H of G which have the property that, for each or each free group H, the reduced rank of H is defined as Set D is a finite subgroup of G with, for all We are interested in precise bounds for σ. If every element of is cyclic, then σ=0. We henceforth assume that some element of has rank two. In the case where G=F and, hence, θ=1, Hanna Neumann and Walter Neumann proved that σ[1,2] and it is a famous conjecture that σ=1, called the Strengthened Hanna Neumann Conjecture. For the general case, we proved that σ[1,2] and if G has 2-torsion then σ=2. We conjectured that if G is 2-torsion-free then σ=1. In this article, we prove the following implications which show that under certain circumstances σ<2. If G is 2-torsion-free and has 3-torsion, then σ≤8/7. If G is 2-torsion-free and 3-torsion-free and has 5-torsion, then σ≤9/5. If p is an odd prime number and G=Cp*Cp, then. In particular, if G=C3*C3 then σ=1, and if G=C5*C5 then σ≤1.52.

Original languageEnglish (US)
Pages (from-to)223-248
Number of pages26
JournalIllinois Journal of Mathematics
Issue number1
StatePublished - 2010
Externally publishedYes

ASJC Scopus subject areas

  • General Mathematics


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