nLab hard Lefschetz theorem

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For (X,ω)(X,\omega) a Kähler manifold the operation of forming the wedge product with the symplectic form ωΩ 1,1(X)\omega \in \Omega^{1,1}(X) induces on de Rham cohomology the Lefschetz operator

L:H k(X)H k+2(X). L \;\colon\; H^k(X) \longrightarrow H^{k+2}(X) \,.

The hard Lefschetz theorem asserts that if XX is compact with complex dimension dim (X)=ddim_{\mathbb{C}}(X)= d, then for all k0k \geq 0 the kkth power of the LL-operation induces an isomorphism

L k:H dk(X)H d+k(X). L^k \;\colon\; H^{d-k}(X) \stackrel{\simeq}{\longrightarrow} H^{d+k}(X) \,.

Using that ω\omega is a (1,1)(1,1)-form this means equivalently in terms of Dolbeault cohomology that for all (p+q)d(p+q) \leq d we have an isomorphism

L d(p+q):H p,q(X)H dq,dp(X). L^{d-(p+q)} \;\colon\; H^{p,q}(X) \stackrel{\simeq}{\longrightarrow} H^{d-q,d-p}(X) \,.

This exhibits the symmetry of the Hodge diamond? under reflection about the horizontal diagonal.

The hard Lefschetz theorem induces the Lefschetz decomposition (see there) of the de Rham cohomology of XX.

The hard Leftschetz theorem also applies to other objects, for instance, to matroids (see Huh 22).

References

In the generality of matroids:

Last revised on August 6, 2022 at 10:38:35. See the history of this page for a list of all contributions to it.