Khler Manifold

In mathematics, a Khler manifold is a complex manifold which also carries a Riemannian metric and a symplectic form on the underlying real manifold in such a way that the three structures (complex, Riemannian, and symplectic) are all mutually compatible. Khler manifolds can thus be thought of as Riemannian manifolds and symplectic manifolds in a natural way. Khler manifolds are named for the mathematician Erich Khler and are important in algebraic geometry.

Definition

A Khler metric on a complex manifold M is a hermitian metric on the complexified tangent bundle TM \otimes \mathbf C satisfying a condition that has several equivalent characterizations (the most geometric being that parallel transport gives rise to complex-linear mappings on the tangent spaces). In terms of local coordinates it is specified in this way: if
h = \sum h_{i\bar j}\; dz^i \otimes d \bar z^j
is the hermitian metric, then the associated Khler form (defined up to a factor of i/2) by
\omega = \sum h_{i\bar j}\; dz^i \wedge d \bar z^j
is closed: that is, dω = 0. If M carries such a metric it is called a Khler manifold.

Examples

  1. Complex Euclidean space Cn with the standard Hermitian metric is a Khler manifold.
  2. A complex torus, given by Cn/Λ for some lattice Λ, forms a compact Khler manifold with the natural metric.
  3. Every Riemann surface is a Khler manifold, since the condition for ω to be closed is trivial in 2 (real) dimensions.
  4. Complex projective space CPn has a natural Khler metric called the Fubini-Study metric. It is essentially determined by the condition that it be invariant under the action of the unitary group (of dimension one larger, acting on the complex vector space giving rise to the projective space).
  5. Any complex submanifold of a Khler manifold is Khler. In particular, any complex manifold that can be embedded in Cn or CPn is Khler.
  6. The restriction properties of the Fubini-Study metric mean that non-singular projective complex algebraic varieties carry Khler metrics. This is fundamental to their analytic theory.
An important subclass of Khler manifolds are Calabi-Yau manifolds.

 

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