On the Cauchy problem for differential operators with double characteristics, transition from effective to non-effective characteristics

TL;DR

This paper investigates the well-posedness of the Cauchy problem for hyperbolic operators with double characteristics transitioning from non-effective to effective hyperbolicity, under specific geometric and symbol conditions.

Contribution

It establishes conditions ensuring well-posedness in all Gevrey classes for such operators, extending understanding of hyperbolic equations with changing characteristic types.

Findings

Proves well-posedness under bounded ratio of imaginary part of subprincipal symbol to eigenvalue.

Shows strict positivity of combined real part of subprincipal symbol and modulus of imaginary eigenvalue.

Addresses the case with no bicharacteristic tangent to the double characteristic manifold.

Abstract

We discuss the well-posedness of the Cauchy problem for hyperbolic operators with double characteristics which changes from non-effectively hyperbolic to effectively hyperbolic, on the double characteristic manifold, across a submanifold of codimension 1. We assume that there is no bicharacteristic tangent to the double characteristic manifold and the spatial dimension is 2. Then we prove the well-posedness of the Cauchy problem in all Gevrey classes assuming, on the double characteristic manifold, that the ratio of the imaginary part of the subprincipal symbol to the real eigenvalue of the Hamilton map is bounded and that the sum of the real part of the subprincipal symbol and the modulus of the imaginary eigenvalue of the Hamilton map is strictly positive.