In this paper we present a number of four-dimensional neutral signature exact solutions for which all of the polynomial scalar curvature invariants vanish (VSI spaces) or are all constant (CSI spaces), which are of relevence in current theoretical physics.
@article{arxiv.1104.3799,
title = {4D neutral signature VSI and CSI spaces},
author = {A. Alcolado and A. MacDougall and A. Coley and S. Hervik},
journal= {arXiv preprint arXiv:1104.3799},
year = {2015}
}