The separability ‘‘theorem’’ in terms of distributions with discussion of electromagnetic scattering theory

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Abstract
The separability theorem states that, given a linear partial differential equation and special coordinates allowing to find a family of separated solutions, all solutions of physical interest of the equations can be obtained from linear combinations of the separated solutions. In developing the theory of interaction between an infinite cylinder and a Gaussian beam, it has been recently observed that the theorem may fail in terms of functions. In this paper, it is shown that the separability theorem is recovered if solutions are expressed in terms of distributions instead of in terms of functions. Relevance to light scattering theory is discussed.