Almost Complex and Almost Product Einstein Manifolds from a Variational Principle

TL;DR

The paper demonstrates that a variational principle applied to certain nonlinear metric-affine Lagrangians yields almost-product and almost-complex Einstein structures, with characterizations and examples provided.

Contribution

It introduces a variational approach to derive almost-product and almost-complex Einstein manifolds from nonlinear metric-affine Lagrangians, including new characterizations and examples.

Findings

Derivation of almost-product Einstein structures from variational principles

Characterization of anti-K"ahler Einstein manifolds

Examples illustrating the theoretical results

Abstract

It is shown that the first order (Palatini) variational principle for a generic nonlinear metric-affine Lagrangian depending on the (symmetrized) Ricci square invariant leads to an almost-product Einstein structure or to an almost-complex anti-Hermitian Einstein structure on a manifold. It is proved that a real anti-Hermitian metric on a complex manifold satisfies the K\"ahler condition on the same manifold treated as a real manifold if and only if the metric is the real part of a holomorphic metric. A characterisation of anti-K\"ahler Einstein manifolds and almost-product Einstein manifolds is obtained. Examples of such manifolds are considered.