  It has been known through some examples that parameters of an electromagnetic
medium can be so defined that there is no dispersion equation (Fresnel
equation) to restrict the choice of the wave vector of a plane wave in such a
medium, i.e., that the dispersion equation is satisfied identically for any
wave vector. In the present paper, a more systematic study to define classes of
media with no dispersion equation is attempted. The analysis makes use of
coordinate-free four-dimensional formalism in terms of multivectors, multiforms
and dyadics.
