On hyperbolicity and Gevrey well-posedness. Part one: the elliptic case

TL;DR

This paper demonstrates that the Cauchy problem for certain elliptic quasi-linear PDE systems is ill-posed in Gevrey spaces, with solutions exhibiting exponential growth and instability over long times depending on the initial spectrum.

Contribution

It establishes ill-posedness in Gevrey spaces for elliptic systems, extending previous results and analyzing the long-time behavior of solutions with high oscillations.

Findings

Ill-posedness in Gevrey spaces depends on initial spectrum.

Instability manifests as exponential growth of solutions.

Instability time scales as a power of frequency in Gevrey spaces.

Abstract

In this paper we prove that the Cauchy problem for first-order quasi-linear systems of partial differential equations is ill-posed in Gevrey spaces, under the assumption of an initial ellipticity. The assumption bears on the principal symbol of the first-order operator. Ill-posedness means instability in the sense of Hadamard, specifically an instantaneous defect of H\"older continuity of the flow from $G^{\sigma}$ to $L^2$, where $\sigma\in(0,1)$ depends on the initial spectrum. Building on the analysis carried out by G. M\'etivier [\textit{Remarks on the well-posedness of the nonlinear Cauchy problem}, Contemp. Math. 2005], we show that ill-posedness follows from a long-time Cauchy-Kovalevskaya construction of a family of exact, highly oscillating, analytical solutions which are initially close to the null solution, and which grow exponentially fast in time. A specific difficulty resides in the observation time of instability. While in Sobolev spaces, this time is logarithmic in the frequency, in Gevrey spaces it is a power of the frequency. In particular, in Gevrey spaces the instability is recorded much later than in Sobolev spaces.