Counting homomorphisms onto finite solvable groups

We present a method for computing the number of epimorphisms from a finitely presented group G to a finite solvable group Γ, which generalizes a formula of Gaschütz. Key to this approach are the degree 1 and 2 cohomology groups of G, with certain twisted coefficients. As an application, we count low-index subgroups of G. We also investigate the finite solvable quotients of the Baumslag-Solitar groups, the Baumslag parafree groups, and the Artin braid groups.