Origin
The first mention of the second identity for a crossed module seems to be in footnote 25 on p. 422 of Whitehead's 1941 paper cited below, while the term `crossed module' is introduced in his 1946 paper cited below. These ideas were well worked up in his 1949 paper `Combinatorial homotopy II', which also introduced the important idea of a free crossed module.
Read more about this topic: Crossed Module
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