We show that, in any Mal'tsev (and a fortiori protomodular) category E, not only the fibre Grd_X E of internal groupoids above the object X is a naturally Mal'tsev category, but moreover it shares with the category Ab of abelian groups the property following which the domain of any split epimorphism is isomorphic with the direct sum of its codomain with its kernel. This allows us to point at a new class of ``non-pointed additive'' categories which is necessarily protomodular. Actually this even gives rise to a larger classification table of non-pointed additive categories which gradually take place between the class of naturally Mal'tsev categories and the one of essentially affine categories. As an application, when furthermore the ground category E is efficiently regular, we get a new way to produce Baer sums in the fibres Grd_X E and, more generally, in the fibres n-Grd_X E.
Keywords: Mal'tsev, protomodular, naturally Mal'tsev categories; internal group; Baer sum; long cohomology sequence
2000 MSC: 18E05,18E10, 18G60, 18C99, 08B05
Theory and Applications of Categories,
Vol. 20, 2008,
No. 4, pp 48-73.
http://www.tac.mta.ca/tac/volumes/20/4/20-04.dvi
http://www.tac.mta.ca/tac/volumes/20/4/20-04.ps
http://www.tac.mta.ca/tac/volumes/20/4/20-04.pdf
ftp://ftp.tac.mta.ca/pub/tac/html/volumes/20/4/20-04.dvi
ftp://ftp.tac.mta.ca/pub/tac/html/volumes/20/4/20-04.ps