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Malcev 1949 (devoted to the study of the coset spaces of nilpotent Lie groups) exhibited an equivalence between the category of torsion free radicable nilpotent finite rank groups and the category of finite dimensional, nilpotent rational Lie algebras. This involves a completion construction which is used also in the general formulation of Hausdorff series (cf. Bourbaki) which takes values in the Mal’cev completion of the universal enveloping algebra on two generators.
(There are several variants of the definition.)
Mal’cev completion is a left adjoint functor to the embedding of the category of uniquely divisible nilpotent groups into the category of nilpotent groups.
Hausdorff series, rational homotopy theory, surgery, combinatorial group theory
A. I. Mal'cev: On a class of homogeneous spaces, Izvestiya Akad. Nauk. SSSR. Ser. Mat. 13 (1949) 9–32 [MR28842]
Russian original: А. И. Мальцев, Об одном классе однородных пространств, Известия Академии наук СССР. Серия математическая 13 1 (1949) 9–32 [mathnet:izv3161]
Nicolas Bourbaki, Lie groups and Lie algebras
Jaume Amorós, On the Malcev completion of Kähler groups, Commentarii Mathematici Helvetici 71, n. 1, 192-212, doi alg-geom/9410013
Robert B. Warfield, The Malcev correspondence, Ch. 12 in Nilpotent Groups, Springer Lec. Notes in Math. 513, 1976, 104-111, doi
Bohumil Cenkl, Richard Porter, Mal’cev’s completion of a group and differential forms, MR628342, euclid
Benoit Fresse, Operads & Grothendieck-Teichmüller groups - draft document, pdf
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