A concept arising from the concept of a -module (see Module). A group (not necessarily Abelian) with a group of operators and homomorphism such that for any and all ,
is called a crossed -module. is a -module (i.e. its underlying group is Abelian) if and only if ().
Crossed modules. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Crossed_modules&oldid=11880