Abstract. The tensor identity a, curl E. curl F = Ial(grad div o1-I, F-div a- ' , grad F) where a is a constant and nonsingular matrix and F is a properly regular vector field, is derived and used to discuss the analogies between second order equations for fields in an anisotropic medium and the Helmholtz equation. The scalar equation for fields in uniform media is also considered; it is shown that this equation is of fourth order and for plane waves in an unbounded medium it is reduced to the generalized Fresnel equation. 1
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