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We show that every algebraically-central extension in a Mal'tsev variety - that is, every surjective homomorphism $f : A \longrightarrow B$ whose kernel-congruence is contained in the centre of $A$, as defined using the theory of commutators - is also a central extension in the sense of categorical Galois theory; this was previously known only for varieties of $\Omega$-groups, while its converse is easily seen to hold for any congruence-modular variety.
@article{TAC_2000_7_a9, author = {G. Janelidze and G.M. Kelly}, title = {Central extensions in {Mal'tsev} varieties}, journal = {Theory and applications of categories}, pages = {219--226}, publisher = {mathdoc}, volume = {7}, year = {2000}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2000_7_a9/} }
G. Janelidze; G.M. Kelly. Central extensions in Mal'tsev varieties. Theory and applications of categories, Tome 7 (2000), pp. 219-226. http://geodesic.mathdoc.fr/item/TAC_2000_7_a9/