Graphs, free groups and the Hanna Neumann conjecture
arXiv:math/0701214
Abstract
A new bound for the rank of the intersection of finitely generated subgroups of a free group is given, formulated in topological terms, and very much in the spirit of Stallings. The bound is a contribution to (although unfortunately not a solution of) the strengthened Hanna Neumann conjecture.
J. Group Theory (to appear)