Co-rank of weakly parafree 3-manifold groups

Abstract

Recall that a group is called large if it has a finite index subgroup which surjects onto a non-abelian free group. By work of Agol and Cooper-Long-Reid, most 3-manifold groups are large; in particular, the fundamental groups of hyperbolic 3-manifolds are large. In previous work, the first author gave examples of closed, hyperbolic 3-manifolds with arbitrarily large first homology rank but whose fundamental groups do not surject onto a non-abelian free group. We call a group very large if it surjects onto a non-abelian free group. In this paper, we consider the question of whether the fundamental groups of homology handlebodies – which are very close to being free – are very large. We show that the fundamental group of W. Thurston’s tripus manifold, is not very large; it is known to be weakly parafree by Stallings’ Theorem and large by the work of Cooper–Long–Reid since the tripus is a hyperbolic manifold with totally geodesic boundary. It is still unknown if a 3-manifold group that is weakly parafree of rank at least 3 must be very large. However, we more generally consider the co-rank of the fundamental group, also known as the cut number of the manifold. For each integer $g\ge 1$ we construct a homology handlebody $Y_g$ of genus $g$ whose group has co-rank equal to $r(g)$, where $r(g)=g/2$ for $g$ even and $r(g)=(g+1)/2$ for $g$ odd. That is, these groups are weakly parafree of rank g and surject onto a free group of rank roughly half of g but no larger.