Semi-Riemannian manifolds with linear differential conditions on the curvature

TL;DR

This paper investigates semi-Riemannian manifolds satisfying linear differential conditions on curvature, providing explicit examples, exploring Lorentzian cases relevant to physics, and opening new research directions.

Contribution

It introduces new families of semi-Riemannian manifolds with differential curvature conditions, including explicit examples across signatures and applications to Lorentzian geometry.

Findings

Existence of all types of such manifolds demonstrated with explicit examples

Special focus on Lorentzian manifolds and their physical relevance

Connections to Gauss-Bonnet gravity and Penrose limits

Abstract

Semi-Riemannian manifolds that satisfy (homogeneous) linear differential conditions of arbitrary order on the curvature are analyzed. They include, in particular, the spaces with (higher-order) recurrent curvature, (higher-order) symmetric spaces, as well as entire new families of semi-Riemannian manifolds rarely, or never, considered before in the literature --such as the spaces whose derivative of the Riemann tensor field is recurrent, among many others. Definite proof that all types of such spaces do exist is provided by exhibiting explicit examples of all possibilities in all signatures, except in the Riemannian case with a positive definite metric. Several techniques of independent interest are collected and presented. Of special relevance is the case of Lorentzian manifolds, due to its connection to the physics of the gravitational field. This connection is discussed with particular emphasis on Gauss-Bonnet gravity and in relation with Penrose limits. Many new lines of research open up and a handful of conjectures, based on the results found hitherto, is put forward.