nLab Maschke's theorem





(Maschke’s theorem) Given a finite group GG and a field kk, the group algebra k[G]k[G] is semisimple iff the characteristic of kk does not divide order of the group.

If the base kk is just a commutative unital ring, then there is the following statement:


If |G|1 k|G|1_k is invertible in kk, then an exact sequence of k[G]k[G]-modules splits iff it splits after applying the forgetful functor from k[G]k[G]-modules to kk-modules (and the splitting in k[G]Mod{}_{k[G]}Mod can be functorially constructed from the splitting in kMod{}_{k}Mod).

If kk is a field, it follows that k[G]k[G] is semisimple, so that Thm. can be understood as a generalization of Maschke’s theorem . This is also one of the motivations for the concept of separable functors.

The importance of the classical Maschke’s theorem is that much is known about the structure of semisimple rings (starting with, e.g., Wedderburn's theorem).


See also:

Last revised on May 19, 2023 at 12:07:24. See the history of this page for a list of all contributions to it.