Radial conditions for global compact finite propagation for divergence-form elliptic operators

Abstract

We study global compact finite propagation (GCFP) for the wave equation $u_{tt}=\operatorname{div}(A(x)\nabla u)$ on $\mathbb{R}^d$, that is, the property that a disturbance starting in a compact set cannot reach spatial infinity in finite time. For smooth strictly positive matrices $A$ we show that GCFP implies $\int_1^\infty a_*(r)^{-1/2}\,dr=\infty$, where $a_*(r)$ is the minimum over the sphere of radius $r$ of $\langle Aω,ω\rangle$; the proof uses the equivalence of GCFP with completeness of the Riemannian metric $A^{-1}$, of which we prove the necessary direction for the cosine family of the Friedrichs extension. We also prove that the analogous divergence condition with $a^*(r)$, the maximum over the sphere, is sufficient, and that this holds for measurable coefficients $A\in L^1_{\mathrm{loc}}$ with $A\ge I$. Finally, we construct a locally bounded measurable matrix with $a_*=a^*$ for which GCFP holds although the integral converges, so the necessary condition is not stable under the loss of continuity.