Compact massive-Laplacian resolvent
= Compact massive-Laplacian resolvent
The solution operator $(-\Delta+m^2)^{-1}:L^2(U)\to L^2(U)$ is compact on every bounded open set because it maps boundedly into $H_0^1(U)$ and the <Rellich-Kondrashov compactness theorem for H01> embeds that space compactly into $L^2(U)$.