Relative Amenability and Relative Soficity.

We define a notion of relative soficity for countable groups with respect to a family of groups. A group is sofic if and only if it is relative sofic with respect to the family consisting only of the trivial group. If a group is relatively sofic with respect to a family of sofic groups, then the group is sofic. Using this notion we generalize a theorem of Elek and Szabo on the soficity of an extension of sofic groups by amenable groups. In particular we prove that groups that are relatively sofic with respect to a family of sofic groups are sofic and more importantly extensions of sofic groups by residually amenable groups are sofic. We also construct examples of relatively amenable groups with respect to an infinite family of subgroups but not with respect to any finite subfamily of the subgroups. As an application of these constructions we show that Deligneu0027s central extension groups are sofic.