  We develop some new aspects of cohomology in the context of semi-abelian
categories: we establish a Hochschild-Serre 5-term exact sequence extending the
classical one for groups and Lie algebras; we prove that an object is perfect
if and only if it admits a universal central extension; we show how the second
Barr-Beck cohomology group classifies isomorphism classes of central
extensions; we prove a universal coefficient theorem to explain the
relationship with homology.
