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
.