Skrypnik, W. I. “On Polymer Expansions for Equilibrium Systems of Oscillators With Ternary Interaction”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 53, no. 11, Nov. 2001, pp. 1532-44, https://umj.imath.kiev.ua/index.php/umj/article/view/4373.