complex geometry

# Contents

## Idea

An $n$-dimensional Calabi-Yau variety is an $n$-dimensional Kähler manifold with (holomorphically, rather than topologically) trivial canonical bundle. This is equivalent to saying that it is real Riemannian manifold of even dimension $2 N$ which has special holonomy in the subgroup $SU(N)\subset O(2 N, \mathbb{R})$.

For compact Kähler manifolds, Yau's theorem? (also known as the Calabi conjecture?) implies that the above conditions are also equivalent to the vanishing of the first Chern class.

Is it also true for non-compact?

Note that $c_1(X) = 0$ implies in general that the canonical bundle is topologically trivial. But if $X$ is a compact Kähler manifold, $c_1(X) = 0$ implies further that the canonical bundle is holomorphically trivial.

## Generalized Definition

A Calabi-Yau variety can be described algebraically as a smooth proper variety $X$ of dimension $n$ over a field $k$ (not necessarily algebraically closed and not necessarily of characteristic $0$) in which $\omega_X=\wedge^n\Omega^1\simeq \mathcal{O}_X$ and also $H^j(X, \mathcal{O}_X)=0$ for all $1\leq j \leq n-1$.

If the base field is $\mathbb{C}$, then one can form the analyticification of $X$ and obtain a compact manifold that satisfies the first given definition.

## Examples

classification of special holonomy manifolds by Berger's theorem:

G-structurespecial holonomydimensionpreserved differential form
Kähler manifoldU(k)$2k$Kähler forms
Calabi-Yau manifoldSU(k)$2k$
hyper-Kähler manifoldSp(k)$4k$
G2 manifoldG2$7$associative 3-form
Spin(7) manifoldSpin(7)8Cayley form

Revised on June 13, 2013 21:07:26 by Urs Schreiber (131.174.43.123)