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In class field theory, the Takagi existence theorem states that for any number field ''K'' there is a one-to-one inclusion reversing correspondence between the finite abelian extensions of ''K'' (in a fixed algebraic closure of ''K'') and the generalized ideal class groups defined via a modulus of ''K''. It is called an existence theorem because a main burden of the proof is to show the existence of enough abelian extensions of ''K''. ==Formulation== Here a modulus (or ''ray divisor'') is a formal finite product of the valuations (also called primes or places) of ''K'' with positive integer exponents. The archimedean valuations that might appear in a modulus include only those whose completions are the real numbers (not the complex numbers); they may be identified with orderings on ''K'' and occur only to exponent one. The modulus ''m'' is a product of a non-archimedean (finite) part ''m''''f'' and an archimedean (infinite) part ''m''∞. The non-archimedean part ''m''''f'' is a nonzero ideal in the ring of integers ''O''''K'' of ''K'' and the archimedean part ''m''∞ is simply a set of real embeddings of ''K''. Associated to such a modulus ''m'' are two groups of fractional ideals. The larger one, ''I''''m'', is the group of all fractional ideals relatively prime to ''m'' (which means these fractional ideals do not involve any prime ideal appearing in ''m''''f''). The smaller one, ''P''''m'', is the group of principal fractional ideals (''u''/''v'') where ''u'' and ''v'' are nonzero elements of ''O''''K'' which are prime to ''m''''f'', ''u'' ≡ ''v'' mod ''m''''f'', and ''u''/''v'' > 0 in each of the orderings of ''m''∞. (It is important here that in ''P''''m'', all we require is that some generator of the ideal has the indicated form. If one does, others might not. For instance, taking ''K'' to be the rational numbers, the ideal (3) lies in ''P''4 because (3) = (−3) and −3 fits the necessary conditions. But (3) is not in ''P''4∞ since here it is required that the ''positive'' generator of the ideal is 1 mod 4, which is not so.) For any group ''H'' lying between ''I''''m'' and ''P''''m'', the quotient ''I''''m''/''H'' is called a ''generalized ideal class group''. It is these generalized ideal class groups which correspond to abelian extensions of ''K'' by the existence theorem, and in fact are the Galois groups of these extensions. That generalized ideal class groups are finite is proved along the same lines of the proof that the usual ideal class group is finite, well in advance of knowing these are Galois groups of finite abelian extensions of the number field. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Takagi existence theorem」の詳細全文を読む スポンサード リンク
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