On a further condition in the macroscopic extended model for ultrarelativistic gases

An exact macroscopic extended model, with many moments, for ultrarelativistic gas has been recently proposed in literature. However, a further condition has not been imposed, even if it is evident in the case of a charged gas and when the electromagnetic field acts as an external force; in the present paper we exploit it and prove that it results in many identities and in residual conditions which allow to determine the arbitrary single variable functions present in the general theory. The result is that they are polynomials determined except for a corresponding number of constants. These are arbitrary constants, so that the macroscopic model remains still more general than the kinetic model.