Regular Cocycles and Biautomatic Structures

Abstract

Let $E$ be a virtually central extension of the group $G$ by a finitely generated abelian group $A$. We show that $E$ carries a biautomatic structure if and only if $G$ has a biautomatic structure $L$ for which the cohomology class of the extension is represented by an $L$-regular cocycle. Moreover, a cohomology class is $L$-regular if some multiple of it is or if its restriction to some finite index subgroup is. We also show that the entire second cohomology of a Fuchsian group is regular, so any virtually central extension is biautomatic. In particular, if the fundamental group of a Seifert fibered 3-manifold is not virtually nilpotent then it is biautomatic. ECHLPT had shown automaticity in this case and in an unpublished 1992 preprint Gersten constructed a biautomatic structure for circle bundles over hyperbolic surfaces and asked if the same could be done for these Seifert fibered 3-manifolds.