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In mathematics, specifically the area of algebraic number theory, a cubic field is an algebraic number field of degree three. ==Definition== If ''K'' is a field extension of the rational numbers Q of degree () = 3, then ''K'' is called a cubic field. Any such field is isomorphic to a field of the form : where ''f'' is an irreducible cubic polynomial with coefficients in Q. If ''f'' has three real roots, then ''K'' is called a totally real cubic field and it is an example of a totally real field. If, on the other hand, ''f'' has a non-real root, then ''K'' is called a complex cubic field. A cubic field ''K'' is called a cyclic cubic field, if it contains all three roots of its generating polynomial ''f''. Equivalently, ''K'' is a cyclic cubic field if it is a Galois extension of Q, in which case its Galois group over Q is cyclic of order three. This can only happen if ''K'' is totally real. It is a rare occurrence in the sense that if the set of cubic fields is ordered by discriminant, then the proportion of cubic fields which are cyclic approaches zero as the bound on the discriminant approaches infinity.〔Harvey Cohn computed an asymptotic for the number of cyclic cubic fields , while Harold Davenport and Hans Heilbronn computed the asymptotic for all cubic fields .〕 A cubic field is called a pure cubic field, if it can be obtained by adjoining the real cube root of a cubefree positive integer ''n'' to the rational number field Q. 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「cubic field」の詳細全文を読む スポンサード リンク
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