Time analyticity for the heat equation and Navier-Stokes equations

TL;DR

This paper establishes time analyticity for solutions of the heat and Navier-Stokes equations in the whole space, providing sharp conditions and novel proofs that do not require decay conditions or boundary assumptions.

Contribution

It proves the first general pointwise time analyticity result for the Navier-Stokes equations in all dimensions using purely real variable methods.

Findings

Sharp solvability condition for backward heat equation

Time analyticity criteria down to initial time

First general pointwise time analyticity result for Navier-Stokes

Abstract

We prove the analyticity in time for solutions of two parabolic equations in the whole space, without any decaying or vanishing conditions. One of them involves solutions to the heat equation of exponential growth of order $2$ on $\M$. Here $\M$ is $\R^d$ or a complete noncompact manifold with Ricci curvature bounded from below by a constant. An implication is a sharp solvability condition for the Cauchy problem of the backward heat equation, which is a well known ill-posed problem. Another implication is a sharp criteria for time analyticity of solutions down to the initial time. The other pertains bounded mild solutions of the incompressible Navier-Stokes equations in the whole space. There are many long established analyticity results for the Navier-Stokes equations. See for example \cite{Ka:1} and \cite{FT:1} for spatial and time analyticity in certain integral sense, \cite{CN:1} for pointwise space-time analyticity of 3 dimensional solutions to the Cauchy problem, and also the pointwise time analyticity results of \cite{Ma:1} and \cite{Gi:1} under zero boundary condition. Our result seems to be the first general pointwise time analyticity result for the Cauchy problem for all dimensions, whose proof involves only real variable method. The proof involves a method of algebraically manipulating the integral kernel, which appears applicable to other evolution equations.