Symmetric cohomology of groups

We investigate the relationship between the symmetric, exterior and classical cohomologies of groups. The first two theories were introduced respectively by Staic and Zarelua. We show in particular, that there is a map from exterior cohomology to symmetric cohomology which is a split monomorphism in general and an isomorphism in many cases, but not always. We introduce two spectral sequences which help to explain the realtionship between these cohomology groups. As a sample application we obtain that symmetric and classical cohomologies are isomorphic for torsion free groups

On the centre of crossed modules of groups

Crossed modules are algebraic models of homotopy 2-types and hence have $\pi_1$ and $\pi_2$. We propose a definition of the centre of a crossed module whose essential invariants can be computed via the group cohomology $H^i(\pi_1,\pi_2)$. This definition therefore has much nicer properties than one proposed by Norrie in the 80s