Continuity of solutions to space-varying pointwise linear elliptic equations

TL;DR

This paper proves the continuity of solutions to space-varying elliptic equations on manifolds under certain conditions, and applies this to the continuity of metrics evolving under Ricci flow on non-smooth initial data.

Contribution

It establishes the L^2-continuity of solutions to elliptic equations with space-dependent coefficients and applies this to the continuity of Ricci flow metrics from rough initial data.

Findings

Solutions are L^2-continuous when coefficients are L^∞-continuous.

Continuity of metrics under Ricci flow from rough initial metrics.

Reduces solution continuity to a homogeneous Kato square root problem.

Abstract

We consider pointwise linear elliptic equations of the form $\mathrm{L}_x u_x = \eta_x$ on a smooth compact manifold where the operators $\mathrm{L}_x$ are in divergence form with real, bounded, measurable coefficients that vary in the space variable $x$. We establish $\mathrm{L}^{2}$-continuity of the solutions at $x$ whenever the coefficients of $\mathrm{L}_x$ are $\mathrm{L}^{\infty}$-continuous at $x$ and the initial datum is $\mathrm{L}^{2}$-continuous at $x$. This is obtained by reducing the continuity of solutions to a homogeneous Kato square root problem. As an application, we consider a time evolving family of metrics $\mathrm{g}_t$ that is tangential to the Ricci flow almost-everywhere along geodesics when starting with a smooth initial metric. Under the assumption that our initial metric is a rough metric on $\mathcal{M}$ with a $\mathrm{C}^{1}$ heat kernel on a "non-singular" nonempty open subset $\mathcal{N}$, we show that $x \mapsto \mathrm{g}_t(x)$ is continuous whenever $x \in \mathcal{N}$.