Schreier theorem on groups which split over free abelian groups

Let G be either a free product with amalgamation A *(C) B or an HNN group A *(C); where C is isomorphic to a free abelian group of finite rank. Suppos e that both A and B have no nontrivial, finitely generated, normal subgroup s of infinite indices. We show that if G contains a finitely generated norm al subgroup N which is neither contained in C nor free, then the index of N in G is finite. Further, as an application of this result, we show that th e fundamental group of a torus sum of 3-manifolds M-1 and M-2, the interior s of which admit hyperbolic structures, have no nontrivial, finitely genera ted, nonfree, normal subgroup of infinite index if each of M-1 and M-2 has at least one nontorus boundary.