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In mathematics, an algebraic cycle on an algebraic variety ''V'' is, roughly speaking, a homology class on ''V'' that is represented by a linear combination of subvarieties of ''V''. Therefore, the algebraic cycles on ''V'' are the part of the algebraic topology of ''V'' that is directly accessible in algebraic geometry. With the formulation of some fundamental conjectures in the 1950s and 1960s, the study of algebraic cycles became one of the main objectives of the algebraic geometry of general varieties. The nature of the difficulties is quite plain: the existence of algebraic cycles is easy to predict, but the current methods of constructing them are deficient. The major conjectures on algebraic cycles include the Hodge conjecture and the Tate conjecture. In the search for a proof of the Weil conjectures, Alexander Grothendieck and Enrico Bombieri formulated what are now known as the standard conjectures of algebraic cycle theory. Algebraic cycles have also been shown to be closely connected with algebraic K-theory. For the purposes of a well-working intersection theory, one uses various equivalence relations on algebraic cycles. Particularly important is the so-called ''rational equivalence''. Cycles up to rational equivalence form a graded ring, the Chow ring, whose multiplication is given by the intersection product. Further fundamental relations include ''algebraic equivalence'', ''numerical equivalence'', and ''homological equivalence''. They have (partly conjectural) applications in the theory of motives. ==Definition== An ''algebraic cycle'' of an algebraic variety or scheme ''X'' is a formal linear combination ''V'' = ∑ ''ni·Vi'' of irreducible reduced closed subschemes. The coefficient ''ni'' is the ''multiplicity'' of ''Vi'' in ''V''. Initially the coefficients are taken to be integers, but rational coefficients are also widely used. Under the correspondence : ↭ (''V'' maps to its generic point (with respect to the Zariski topology), conversely a point maps to its closure (with the reduced subscheme structure)) an algebraic cycle is thus just a formal linear combination of points of ''X''. The group of cycles naturally forms a group ''Z'' *(''X'') graded by the dimension of the cycles. The grading by codimension is also useful, then the group is usually written ''Z'' *(''X''). 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「algebraic cycle」の詳細全文を読む スポンサード リンク
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