Singularities and diffeomorphisms

TL;DR

This paper addresses the complex gauge problem in comparing metrics under diffeomorphisms, introducing a general PDE-based method to find diffeomorphisms fixing a gauge, with applications to Ricci flow.

Contribution

It develops a general approach to solve the gauge problem for metrics via nonlinear PDEs, applicable even without additional structure, and solves a known open problem in Ricci flow.

Findings

Established optimal bounds for the displacement function of the diffeomorphism.

Provided a PDE-based method to fix gauges in metric comparison problems.

Applied techniques to solve an open problem in Ricci flow.

Abstract

Comparing and recognizing metrics can be extraordinarily difficult because of the group of diffeomorphisms. Two metrics, that could even be the same, could look completely different in different coordinates. This is the gauge problem. The general gauge problem is extremely subtle, especially for non-compact spaces. Often it can be avoided if one uses some additional structure of the particular situation. However, in many problems there is no additional structure. Instead we solve the gauge problem directly in great generality. The techniques and ideas apply to many problems. We use them to solve a well-known open problem in Ricci flow. We solve the gauge problem by solving a nonlinear system of PDEs. The PDE produces a diffeomorphism that fixes an appropriate gauge in the spirit of the slice theorem for group actions. We then show optimal bounds for the displacement function of the diffeomorphism.