On the second cohomology group in semi-abelian categories
arXiv:math/0511357 · doi:10.1016/j.jpaa.2007.06.009
Abstract
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.
23 pages, major changes in sections 4 and 6, minor changes elsewhere
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