Mutual orientation of the Poynting vector and the group velocity vector of electromagnetic waves in bigyrotropic medium

The collinearity of the Poynting vector P and the group velocity vector U of electromagnetic waves in a bihyrotropic medium characterized by second rank Hermitian tensors of dielectric and magnetic permittivities is theoretically proved. It is shown that these vectors will be collinear in any anisotropic medium having either Hermitian or symmetric dielectric and magnetic permittivities tensors.