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Capitulation (algebra) : ウィキペディア英語版
Principalization (algebra)

In the mathematical field of algebraic number theory, the concept of principalization refers to a situation when given an extension of algebraic number fields, some ideal (or more generally fractional ideal) of the ring of integers of the smaller field isn't principal but it's extension to the ring of integers of the larger field is. Its study has origins in the work of Ernst Kummer on ideal numbers from the 1840s, who in particular proved that for every algebraic number field there exists an extension number field such that all ideals of the ring of integers of the base field (which can always be generated by at most two elements) become principal when extended to the larger field. In 1897 David Hilbert conjectured that maximal abelian unramified extension of the base field, which was later called the Hilbert class field of the given base field, is such an extension. This conjecture, now known as principal ideal theorem, was proved by Philipp Furtwängler in 1930 after it had been translated from number theory to group theory by Emil Artin in 1929, who made use of his general reciprocity law to establish the reformulation. Since this long desired proof was achieved by means of Artin transfers of non-abelian groups with derived length two, several investigators tried to exploit the theory of such groups further to obtain additional information on the principalization in intermediate fields between the base field and its Hilbert class field. The first contributions in this direction are due to Arnold Scholz and Olga Taussky in 1934, who coined the synonym capitulation for principalization. Another independent access to the principalization problem via Galois cohomology of unit groups is also due to Hilbert and goes back to the chapter on cyclic extensions of number fields of prime degree in his number report, which culminates in the famous Theorem 94.
==Extension of classes==
Let K be an algebraic number field, called the ''base field'', and let L/K be a field extension of finite degree. Let \mathcal_K,\mathcal_K,\mathcal_K and \mathcal_L,\mathcal_L,\mathcal_L denote the ring of integers, the group of nonzero fractional ideals and its subgroup of principal fractional ideals of the fields K,L respectively. Then the extension map of frational ideals \iota_:\ \mathcal_K\to\mathcal_L,\ \mathfrak\mapsto\mathfrak\mathcal_L is an injective group homomorphism. Since \iota_(\mathcal_K)\subseteq\mathcal_L, this map induces the extension homomorphism of ideal class groups
j_:\ \mathcal_K/\mathcal_K\to\mathcal_L/\mathcal_L,\ \mathfrak\mathcal_K\mapsto(\mathfrak\mathcal_L)\mathcal_L.
If there exists a non-principal ideal \mathfrak\in\mathcal_K (i.e. \mathfrak\mathcal_K\ne\mathcal_K) whose extension ideal in L is principal (i.e. \mathfrak\mathcal_L=A\mathcal_Lfor some A\in\mathcal_L and (\mathfrak\mathcal_L)\mathcal_L=(A\mathcal_L)\mathcal_L=\mathcal_L), then we speak about principalization or capitulation in L/K. In this case, the ideal \mathfrak and its class \mathfrak\mathcal_K are said to principalize or capitulate in L. This phenomenon is described most conveniently by the principalization kernel or capitulation kernel, that is the kernel \ker(j_) of the class extension homomorphism.
More generally, let \mathfrak=\mathfrak_0\mathfrak_\infty be a modulus in K, where \mathfrak_0 is nonzero ideal in \mathcal_K and \mathfrak_\infty is a formal product of pair-wise different real infinite primes of K. Then \mathcal__K\mid\alpha\equiv
1\pmod_K(\mathfrak) is the ray modulo \mathfrak, where \mathcal_K(\mathfrak)=\mathcal_K(\mathfrak_0) is the group of nonzero fractional ideals in K relatively prime to \mathfrak_0 and the condition \alpha\equiv1\pmod_0} and v(\alpha)>0 for every real infinite prime v dividing \mathfrak_\infty. Let \mathcal_\le\mathcal_K(\mathfrak), then the group \mathcal_K(\mathfrak)/\mathcal is called generalized ideal class group for \mathfrak. If \mathcal_K(\mathfrak_K)/\mathcal_K and \mathcal_L(\mathfrak_L)/\mathcal_L are generalized ideal class groups such that \mathfrak\mathcal_L\in\mathcal_L(\mathfrak_L) for every \mathfrak\in\mathcal_K(\mathfrak_K) and \mathfrak\mathcal_L\in\mathcal_L for every \mathfrak\in\mathcal_K, then \iota_ induces the extension homomorphism of generalized ideal class groups
j_:\ \mathcal_K(\mathfrak_K)/\mathcal_K\to\mathcal_L(\mathfrak_L)/\mathcal_L,\ \mathfrak\mathcal_K\mapsto(\mathfrak\mathcal_L)\mathcal_L.

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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