Ys. Semenov, PROOF OF A GENERALIZATION OF THE CONJECTURE OF HANNA-NEUMANN FOR VIRTUALLY FREE GROUPS, Sbornik. Mathematics, 187(9-10), 1996, pp. 1561-1575
The article confirms a generalization of a conjecture of Hanna Neumann
for groups having the free product of three groups of order 2 as subg
roups of finite index.