Nearly pseudo-K\"ahler and nearly para-K\"ahler six-manifolds

The subject of this paper is six-dimensional nearly (para-)K\"ahler geometry with pseudo-Riemannian metrics. Firstly, we derive the analogue of the well-known exterior differential system characterising a nearly K\"ahler manifold and prove applications to the automorphism group of a nearly (para-)K\"ahler structure. Secondly, we prove existence and uniqueness results for left-invariant nearly (para-)K\"ahler structures on Lie groups $G \times G$ where $G$ is three-dimensional and simple.