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Answer to Question #23891 in Abstract Algebra for john.george.milnor

Question #23891
Can every finite group be realized as an irreducible linear group?
Consider an irreducible linear groupG &sube;GL(V ), where V is a finite-dimensional vector space overa field k. We claim that Z(G) is cyclic. This willshow, for instance, that the Klein 4-group Z2 &oplus;Z2 cannotbe realized as an irreducible linear group. To prove the claim, consider D =End(kGV ), which is, by Schur&rsquo;s Lemma, a (finite dimensional)division algebra over k. Since every g &isin; Z(G) acts as a kG-automorphism of V, we can think of Z(G) as embedded in D*. The k-algebraF generated by Z(G) in D is a finite-dimensional k-domain,so F is a field. By a well-known theorem in field theory, Z(G)&sube; F* must be a cyclic group.

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