On surface integral representations: validity of Huygens' principle and the equivalence principle in inhomogeneous bianisotropic media

Here we provide the mathematical foundation for general inhomogeneous (even discontinuous) media for the principle that Huygens devised with ingenious foresight over three hundred years ago (1690). We also validate the associated (and often used without a proof) equivalence principle as a natural extension of the isotropic formalism.<
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