The notions of tensor end exterior products modulo $q$ of two crossed $P$-modules, where $q$ is a positive integer and $P$ is a Lie algebra, are introduced and some properties are established. The condition for the existence of a universal $q$-central relative extension of a Lie epimorphism is given and this extension is described as an exterior product modulo $q$.
Homology, Homotopy and Applications, Vol. 1, 1999, No. 9, pp 187-204
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