An approximation theorem of Runge type for the heat equation

Abstract:

If $\Omega$ is an open subset of ${{\mathbf {R}}^{n + 1}}$, the approximation problem is to decide whether every solution of the heat equation on $\Omega$ can be approximated by solutions defined on all of ${{\mathbf {R}}^{n + 1}}$. The necessary and sufficient condition on $\Omega$ which insures this type of approximation is that every section of $\Omega$ taken by hyperplanes orthogonal to the $t$-axis be an open set without “holes,” i.e., whose complement has no compact component. Part of the proof involves the Tychonoff counterexample for the initial value problem.