A canonical structure on the tangent bundle of a pseudo- or para-K\"ahler manifold

TL;DR

This paper shows that the tangent bundle of a pseudo- or para-Kähler manifold naturally inherits a compatible pseudo-Kähler or para-Kähler structure, and explores its curvature properties and geometric implications.

Contribution

It establishes a canonical pseudo-Kähler or para-Kähler structure on the tangent bundle of such manifolds and analyzes its curvature characteristics.

Findings

G is scalar-flat but not Einstein unless g is flat

G has nonpositive or nonnegative Ricci curvature depending on g

G is locally conformally flat only in specific flat or constant curvature cases

Abstract

It is a classical fact that the cotangent bundle $T^* \M$ of a differentiable manifold $\M$ enjoys a canonical symplectic form $\Omega^*$. If $(\M,\j,g,\omega)$ is a pseudo-K\"ahler or para-K\"ahler $2n$-dimensional manifold, we prove that the tangent bundle $T\M$ also enjoys a natural pseudo-K\"ahler or para-K\"ahler structure $(\J,\G,\Omega)$, where $\Omega$ is the pull-back by $g$ of $\Omega^*$ and $\G$ is a pseudo-Riemannian metric with neutral signature $(2n,2n)$. We investigate the curvature properties of the pair $(\J,\G)$ and prove that: $\G$ is scalar-flat, is not Einstein unless $g$ is flat, has nonpositive (resp.\ nonnegative) Ricci curvature if and only if $g$ has nonpositive (resp.\ nonnegative) Ricci curvature as well, and is locally conformally flat if and only if $n=1$ and $g$ has constant curvature, or $n>2$ and $g$ is flat. We also check that (i) the holomorphic sectional curvature of $(\J,\G)$ is not constant unless $g$ is flat, and (ii) in $n=1$ case, that $\G$ is never anti-self-dual, unless conformally flat.