Projective Metrizability and Formal Integrability

TL;DR

This paper investigates conditions under which a spray's geodesics match those of a Finsler space, using differential operators and curvature conditions to establish formal integrability and classify projectively metrizable sprays.

Contribution

It reformulates the projective metrizability problem via a PDE approach, analyzes formal integrability conditions, and characterizes classes of projectively metrizable sprays.

Findings

Symbol of the PDE is involutive.

Curvature tensor determines obstructions to integrability.

Identifies classes of sprays that are projectively metrizable.

Abstract

The projective metrizability problem can be formulated as follows: under what conditions the geodesics of a given spray coincide with the geodesics of some Finsler space, as oriented curves. In Theorem 3.8 we reformulate the projective metrizability problem for a spray in terms of a first-order partial differential operator $P_1$ and a set of algebraic conditions on semi-basic 1-forms. We discuss the formal integrability of $P_1$ using two sufficient conditions provided by Cartan-K\"ahler theorem. We prove in Theorem 4.2 that the symbol of $P_1$ is involutive and hence one of the two conditions is always satisfied. While discussing the second condition, in Theorem 4.3 we prove that there is only one obstruction to the formal integrability of $P_1$, and this obstruction is due to the curvature tensor of the induced nonlinear connection. When the curvature obstruction is satisfied, the projective metrizability problem reduces to the discussion of the algebraic conditions, which as we show are always satisfied in the analytic case. Based on these results, we recover all classes of sprays that are known to be projectively metrizable: flat sprays, isotropic sprays, and arbitrary sprays on 1- and 2-dimensional manifolds. We provide examples of sprays that are projectively metrizable without being Finsler metrizable.