D. Bourn, G. Janelidze

Extensions with Abelian Kernels in Protomodular Categories

As observed by J. Beck, and as we know from M. Barr's and his joint work on triple cohomology, the classical isomorphism Opext $\cong H^2$ that describes group extensions with abelian kernels, can be deduced from the equivalence between such extensions and torsors (in an appropriate sense). The same is known for many other "group-like"' algebraic structures, and now we present a purely-categorical version of that equivalence, essentially by showing that all torsors are extensions with abelian kernels in any pointed protomodular category, and by giving a necessary and sufficient condition for the converse.

Protomodular category, semi-abelian category, Opext functor, extension, torsor.

MSC 2000: 18G50, 18D35, 20J06, 18C10, 18G60