We propose a brief survey devoted to the description of the solutions of Kirkwood-Salsburg equations for correlation functions in the grand canonical ensemble. We establish analytic relationships between the Ruelle operator approach described in detail in [D. Ruelle, Statistical Mechanics. Rigorous Results, Chap. 4, Benjamin, New York (1969)] and the approach developed by Minlos and Pogosyan in [Teor. Mat. Fiz., 31, No. 2, 199 (1977)]. Based on the methods of infinite-dimensional analysis, we propose a more transparent description of the main results.