The so-called sum rule for the extinction cross section has been the subject of several publications. However, it has not been obtained directly from the macroscopic Maxwell equations but rather follows from heuristic causality considerations. It is argued that these causality considerations are, in fact, questionable and do not follow from the fundamental concept of electromagnetic scattering by a particle. Therefore, the resulting sum rule should be considered an unproven hypothesis rather than an outcome of a rigorous derivation from first principles.
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