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A Calabi–Yau manifold, also known as a Calabi–Yau space, is a special type of manifold that is described in certain branches of mathematics such as algebraic geometry. The Calabi–Yau manifold's properties, such as Ricci flatness, also yield applications in theoretical physics. Particularly in superstring theory, the extra dimensions of spacetime are sometimes conjectured to take the form of a 6-dimensional Calabi–Yau manifold, which led to the idea of mirror symmetry. Calabi–Yau manifolds are complex manifolds that are generalizations of K3 surfaces in any number of complex dimensions (i.e. any even number of real dimensions). They were originally defined as compact Kähler manifolds with a vanishing first Chern class and a Ricci-flat metric, though many other similar but inequivalent definitions are sometimes used. They were named "Calabi–Yau spaces" by after who first conjectured that such surfaces might exist, and who proved the Calabi conjecture. ==Definitions== The motivational definition given by Yau is of a compact Kähler manifold with a vanishing first Chern class, that is also Ricci flat.〔Yau and Nadis (2010)〕 Calabi conjectured their existence and Yau proved the conjecture. There are many other definitions of a Calabi–Yau manifold used by different authors, some inequivalent. This section summarizes some of the more common definitions and the relations between them. A Calabi–Yau ''n''-fold or Calabi–Yau manifold of (complex) dimension ''n'' is sometimes defined as a compact ''n''-dimensional Kähler manifold ''M'' satisfying one of the following equivalent conditions: * The canonical bundle of ''M'' is trivial. * ''M'' has a holomorphic ''n''-form that vanishes nowhere. * The structure group of ''M'' can be reduced from U(n) to SU(n). * ''M'' has a Kähler metric with global holonomy contained in SU(n). These conditions imply that the first integral Chern class c1(''M'') of ''M'' vanishes, but the converse is not true. The simplest examples where this happens are hyperelliptic surfaces, finite quotients of a complex torus of complex dimension 2, which have vanishing first integral Chern class but non-trivial canonical bundle. For a compact ''n''-dimensional Kähler manifold ''M'' the following conditions are equivalent to each other, but are weaker than the conditions above, and are sometimes used as the definition of a Calabi–Yau manifold: * ''M'' has vanishing first real Chern class. * ''M'' has a Kähler metric with vanishing Ricci curvature. * ''M'' has a Kähler metric with local holonomy contained in SU(n). * A positive power of the canonical bundle of ''M'' is trivial. * ''M'' has a finite cover that has trivial canonical bundle. * ''M'' has a finite cover that is a product of a torus and a simply connected manifold with trivial canonical bundle. In particular if a compact Kähler manifold is simply connected then the weak definition above is equivalent to the stronger definition. Enriques surfaces give examples of complex manifolds that have Ricci-flat metrics, but their canonical bundles are not trivial so they are Calabi–Yau manifolds according to the second but not the first definition above. Their double covers are Calabi–Yau manifolds for both definitions (in fact K3 surfaces). By far the hardest part of proving the equivalences between the various properties above is proving the existence of Ricci-flat metrics. This follows from Yau's proof of the Calabi conjecture, which implies that a compact Kähler manifold with a vanishing first real Chern class has a Kähler metric in the same class with vanishing Ricci curvature. (The class of a Kähler metric is the cohomology class of its associated 2-form.) Calabi showed such a metric is unique. There are many other inequivalent definitions of Calabi–Yau manifolds that are sometimes used, which differ in the following ways (among others): * The first Chern class may vanish as an integral class or as a real class. * Most definitions assert that Calabi–Yau manifolds are compact, but some allow them to be non-compact. In the generalization to non-compact manifolds, the difference must vanish asymptotically. Here, is the Kähler form associated with the Kähler metric, . * Some definitions put restrictions on the fundamental group of a Calabi–Yau manifold, such as demanding that it be finite or trivial. Any Calabi–Yau manifold has a finite cover that is the product of a torus and a simply-connected Calabi–Yau manifold. * Some definitions require that the holonomy be exactly equal to SU(n) rather than a subgroup of it, which implies that the Hodge numbers ''h''''i'',0 vanish for 0 < i < dim(''M''). Abelian surfaces have a Ricci flat metric with holonomy strictly smaller than SU(2) (in fact trivial) so are not Calabi–Yau manifolds according to such definitions. * Most definitions assume that a Calabi–Yau manifold has a Riemannian metric, but some treat them as complex manifolds without a metric. * Most definitions assume the manifold is non-singular, but some allow mild singularities. While the Chern class fails to be well-defined for singular Calabi–Yau's, the canonical bundle and canonical class may still be defined if all the singularities are Gorenstein, and so may be used to extend the definition of a smooth Calabi–Yau manifold to a possibly singular Calabi–Yau variety. 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「Calabi–Yau manifold」の詳細全文を読む スポンサード リンク
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