  We investigate the nonextensivity and the q-distribution of a relativistic
gas under an external electromagnetic field. We derive a formula expression of
the nonextensive parameter q based on the relativistic generalized Boltzmann
equation, the relativistic q-H theorem and the relativistic version of
q-power-law distribution function in the nonextensive q-kinetic theory. We thus
provide the connection between the parameter 1-q and the differentiation of the
temperature field of the gas as well as the four-potential with respect to time
and space coordinates, and therefore present the nonextensivity for the gas a
clearly physical meaning.
