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Wedderburn's little theorem
In mathematics, Wedderburn's little theorem states that every finite domain is a field. In other words, for finite rings, there is no distinction between domains, skew-fields and fields.
The Artin–Zorn theorem generalizes the theorem to alternative rings: every finite alternative division ring is a field.
== History ==
The original proof was given by Joseph Wedderburn in 1905,〔Lam (2001), (p. 204 )〕 who went on to prove it two other ways. Another proof was given by Leonard Eugene Dickson shortly after Wedderburn's original proof, and Dickson acknowledged Wedderburn's priority. However, as noted in , Wedderburn's first proof was incorrect – it had a gap – and his subsequent proofs appeared only after he had read Dickson's correct proof. On this basis, Parshall argues that Dickson should be credited with the first correct proof.
A simplified version of the proof was later given by Ernst Witt.〔 Witt's proof is sketched below. Alternatively, the theorem is a consequence of the Skolem–Noether theorem by the following argument.〔Theorem 4.1 in Ch. IV of Milne, class field theory, http://www.jmilne.org/math/CourseNotes/cft.html〕 Let ''D'' be a finite division algebra with center ''k''. Let (: ''k'' ) = ''n''2 and ''q'' denote the cardinality of ''k''. Every maximal subfield of ''D'' has ''qn'' elements; so they are isomorphic and thus are conjugate by Skolem–Noether. But a finite group (the multiplicative group of ''D'' in our case) cannot be a union of conjugates of a proper subgroup; hence, ''n'' = 1.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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