Boundary Perturbation Theory for Nonanalytic Perturbations

First-order perturbation formulas are derived that give the change in the eigenvalue of a reactive system due to a perturbation in the exterior shape of the system. In physical terms, this perturbation involves adding a thin layer of arbitrary material to the surface of the unperturbed system (or deleting material past a material discontinuity). From a mathematical viewpoint, the perturbation is sufficiently general to give rise to a nonanalytic behavior of the eigenvalue on the smallness parameter. Both transport theory and the diffusion approximation are treated.