is usually a symmetric operator under the "inner product" formula_38 for vector fields formula_39 and formula_40. (Technically, this unconjugated form is not a true inner product because it is not real-valued for complex-valued fields, but that is not a problem here. In this sense, the operator is not truly Hermitian but is rather complex-symmetric.) This is true whenever the permittivity ε and the magnetic permeability μ, at the given ω, are symmetric 3×3 matrices (symmetric rank-2 tensors) — this includes the common case where they are scalars (for isotropic media), of course. They need "not" be real—complex values correspond to materials with losses, such as conductors with finite conductivity σ (which is included in ε via formula_41)—and because of this the reciprocity theorem does "not" require time reversal invariance. The condition of symmetric ε and μ matrices is almost always satisfied; see below for an exception.