Information AboutSchur's Lemma |
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The original case may have been for Linear Representation s of a finite Group ''G'' over the Complex Number field C (it does not apply to real fields). If ''G'' acts irreducibly on a finite-dimensional complex Vector Space ''V'' through a Group Representation ρ, then the only Linear Transformation s α of ''V'' to itself, such that :αρ(''g'') = ρ(''g'')α for all ''g'' in ''G'', are scalar multiples of the Identity Transformation . Here an ''irreducible representation'' on ''V'' is simply one with no Invariant Subspace s aside from {0} and ''V'' itself (because the Group Algebra is Semisimple ). Since scalar 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 Field s, to Lie Group s and Lie Algebra s, and in Module Theory . In the latter, a theorem commonly called Schur's Lemma states that the Endomorphism Ring End''R''''M'' of any simple ''R''-module ''M'' is a Division Ring . REFERENCE
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