### Speaker

Mariam Pirashvili

### Description

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 80’s.