Skrypnik, W. I. “On the Evolution Operator of the Gradient Diffusion Hierarchy for Plane Rotators”. Ukrains’kyi Matematychnyi Zhurnal 53, no. 12 (December 25, 2001): 1664-1685. Accessed May 18, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4386.