Schur's lemma
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In mathematics, Schur's lemma is now a generic term applied to theorems on the commutant of a module M that is simple. It is named for Issai Schur.
The original case may have been for linear representations of a finite group G over the complex number field C. If G acts irreducibly on a finite-dimensional complex vector space V through a group representation ρ, then the only linear transformations α of V to itself, such that
- αρ(g) = ρ(g)α
for all g in G, are multiples of the identity transformation. Here an irreducible representation on V is simply one with no invariant subspaces aside from {0} and V itself (because the group algebra is semisimple).
Since multiples of the identity transformation trivially commute with all linear transformations, one can say that the import of the lemma is in this case that the commutant of the representation is as small as possible. The condition of irreducibility is necessary because a non-trivial invariant subspace would be the image of a projection operator that would commute with the ρ(g) (see Maschke's theorem).
There are many generalisations: to other fields, to Lie groups and Lie algebras, and in module theory.