Integrability properties for relativistic extended thermodynamics of polyatomic gas

In a recent article (Ann Phys 377:414–445, 2017. https://doi.org/10.1016/ j.aop.2016.12.012), Pennisi and Ruggeri proposed a relativistic extended thermodynamics theory of polyatomic gas. It was achieved by adopting the closure procedure for the generalized moments of a distribution function that, as in the classical case, depends on an additional continuous variable representing the energy of the internal modes of a molecule; this permits the theory to take into account the energy exchange between the translational and the internal modes of a molecule in binary collisions. In this paper the attention will be focused on some integrals appearing in the field equations of the relativistic theory of polyatomic gas and their integrability will be proven. In the last part of the paper we consider the case of a monatomic gas and we evaluate the ultra-relativistic limit of the differential system of balance laws.