What is a double central extension ? (the question was asked by Ronald Brown)
Janelidze, George
Cahiers de Topologie et Géométrie Différentielle Catégoriques, Tome 32 (1991), p. 191-201 / Harvested from Numdam
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     author = {Janelidze, George},
     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},
     volume = {32},
     year = {1991},
     pages = {191-201},
     mrnumber = {1158108},
     zbl = {0762.18003},
     language = {en},
     url = {http://dml.mathdoc.fr/item/CTGDC_1991__32_3_191_0}
}

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) pp. 191-201. http://gdmltest.u-ga.fr/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