ZaviyalovО. I., and PolivanovМ. K. “R-Operation of Bogolyubov—Parasyuk and Operator Expansions in Mass-Free Theories ”. Ukrains’kyi Matematychnyi Zhurnal 44, no. 3 (April 2, 1992): 333-340. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/8102.