Petrina, E. D., and D. Ya. Petrina. “Existence of Equilibrium States of Systems of Hard Spheres in the Boltzmann-Enskog Limit Within the Frame Work of the Grand Canonical Ensemble”. Ukrains’kyi Matematychnyi Zhurnal, vol. 49, no. 1, Jan. 1997, pp. 112–121, https://umj.imath.kiev.ua/index.php/umj/article/view/4990.