  The difference of the resolvents of two Laplacians on a half-space subject to
Robin boundary conditions is studied. In general this difference is not
compact, but it will be shown that it is compact and even belongs to some
Schatten-von-Neumann class, if the coefficients in the boundary condition are
sufficiently close to each other in a proper sense. In certain cases the
resolvent difference is shown to belong even to the same Schatten-von Neumann
class as it is known for the resolvent difference of two Robin Laplacians on a
domain with a compact boundary.
