ZaviyalovО. I., and PolivanovМ. K. “R-Operation of Bogolyubov—Parasyuk and Operator Expansions in Mass-Free Theories ”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 44, no. 3, Apr. 1992, pp. 333-40, https://umj.imath.kiev.ua/index.php/umj/article/view/8102.