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In mathematics, specifically in symplectic topology and algebraic geometry, a quantum cohomology ring is an extension of the ordinary cohomology ring of a closed symplectic manifold. It comes in two versions, called small and big; in general, the latter is more complicated and contains more information than the former. In each, the choice of coefficient ring (typically a Novikov ring, described below) significantly affects its structure, as well. While the cup product of ordinary cohomology describes how submanifolds of the manifold intersect each other, the quantum cup product of quantum cohomology describes how subspaces intersect in a "fuzzy", "quantum" way. More precisely, they intersect if they are connected via one or more pseudoholomorphic curves. Gromov–Witten invariants, which count these curves, appear as coefficients in expansions of the quantum cup product. Because it expresses a structure or pattern for Gromov–Witten invariants, quantum cohomology has important implications for enumerative geometry. It also connects to many ideas in mathematical physics and mirror symmetry. In particular, it is ring-isomorphic to Floer homology. Throughout this article, ''X'' is a closed symplectic manifold with symplectic form ω. ==Novikov ring== Various choices of coefficient ring for the quantum cohomology of ''X'' are possible. Usually a ring is chosen that encodes information about the second homology of ''X''. This allows the quantum cup product, defined below, to record information about pseudoholomorphic curves in ''X''. For example, let : be the second homology modulo its torsion. Let ''R'' be any commutative ring with unit and Λ the ring of formal power series of the form : where * the coefficients come from ''R'', * the are formal variables subject to the relation , * for every real number ''C'', only finitely many ''A'' with ω(''A'') less than or equal to ''C'' have nonzero coefficients . The variable is considered to be of degree , where is the first Chern class of the tangent bundle ''TX'', regarded as a complex vector bundle by choosing any almost complex structure compatible with ω. Thus Λ is a graded ring, called the Novikov ring for ω. (Alternative definitions are common.) 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「quantum cohomology」の詳細全文を読む スポンサード リンク
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