Geometric orbifolds with torsion free derived subgroup

A geometric orbifold of dimension d is the quotient space S = X/K, where (X,G) is a geometry of dimension d and K < G is a co-compact discrete subgroup. In this case {ie38-01} is called the orbifold fundamental group of S. In general, the derived subgroup K’ of K may have elements acting with fixed points; i.e., it may happen that the homology cover M_S = X/K’ of S is not a geometric manifold: it may have geometric singular points. We are concerned with the problem of deciding when K′ acts freely on X; i.e., when the homology cover M _S is a geometric manifold. In the case d = 2 a complete answer is due to C. Maclachlan. In this paper we provide necessary and sufficient conditions for the derived subgroup S to act freely in the case d = 3 under the assumption that the underlying topological space of the orbifold K is the 3-sphere S ³.