What is a double central extension ? (the question was asked by Ronald Brown)
Cahiers de Topologie et Géométrie Différentielle Catégoriques, Tome 32 (1991) no. 3, pp. 191-201.
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     title = {What is a double central extension ? (the question was asked by {Ronald} {Brown)}},
     journal = {Cahiers de Topologie et G\'eom\'etrie Diff\'erentielle Cat\'egoriques},
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Janelidze, George. What is a double central extension ? (the question was asked by Ronald Brown). Cahiers de Topologie et Géométrie Différentielle Catégoriques, Tome 32 (1991) no. 3, pp. 191-201. http://www.numdam.org/item/CTGDC_1991__32_3_191_0/

1 R. Brown & G.J. Ellis, 'Hopf formulae for the higher homology of a group', Bull. London Math. Soc. 20 (1988) 124-128. | MR 924238 | Zbl 0611.20032

2 G. Janalidze, 'Pure Galois theory in categories', J. Algebra 132 (1990) 270-286. (First version was available as UCNW Preprint 87.20, Bangor, 1987.) | MR 1061480