Intersections of finitely generated free groups

Abstract

A result of Howson is that two finitely generated subgroups U and V of a free group have finitely generated intersection. Hanna Neumann showed further that, if m, n and N are the ranks of U, V and U ∩ V respectively, then N ≤ 2(m−1)(n−1) + 1, and Burns strengthened this, showing that N ≤ 2(m−1)(n−1) − m + 2 (if m ≤ n). This paper presents a new and simple proof of Burns' result. Further, the graph-theoretical ideas used provide still stronger bounds in certain special cases.