Reconstruction of the connection or metric from some partial information

TL;DR

This paper presents methods to reconstruct the metric tensor and symmetric linear connection of a (pseudo-)Riemannian manifold from partial initial data and curvature components, using differential equations and semigeodesic coordinates.

Contribution

It introduces a generalized approach for reconstructing metrics and connections from partial information, extending previous methods with shorter, more direct proofs.

Findings

Reconstruction of the metric tensor from boundary data and curvature components.

Reconstruction of the symmetric linear connection from initial conditions and curvature.

Shorter, constructive proofs based on classical ODE theory.

Abstract

In a neighborhood of a (positive definite) Riemannian space in which special, semigeodesic, coordinates are given, the metric tensor can be calculated from its values on a suitable hypersurface and some of components of the curvature tensor of type $(1,3)$ in the coordinate domain. Semigeodesic coordinates are a generalization of the well-known Fermi coordinates, that play an important role in mechanics and physics, are widely used in Minkowskian space, and in differential geometry of Riemannian spaces in general. In the present paper, we consider a more general situation. We introduce special pre-semigeodesic charts characterized both geometrically and in terms of the connection, formulate a version of the Peano's-Picard's-Cauchy-like Theorem on existence and uniqueness of solutions of the initial values problems for systems of first-order ordinary differential equations. Then we use the apparatus in a fixed pre-semigeodesic chart of a manifold equipped with the linear symmetric connection. Our aim is to reconstruct, or construct, the symmetric linear connection in some neighborhood from the knowledge of the "initial conditions": the restriction of the connection to a fixed $(n-1)$-dimensional surface $S$ and some of the components of the curvature tensor $R$ in the "volume" (coordinate domain). By analogous methods, we retrieve (or construct) the metric tensor of type $(0,4)$ of a pseudo-Riemannian manifold in a domain of semigeodesic coordinates from the known restriction of the metric to some non-isotropic hypersurface and some of the components of the curvature tensor in the volume. In comparison to other authors, we give shorter proofs of constructive character based on classical results on first order ODEs (ordinary differential equations).