Simulik, V. M. “A Theorem on the Structure of a Complete Set of Conformal-Like Series of Conserved Quantities for Massless Fields”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 49, no. 12, Dec. 1997, pp. 1707–1711, https://umj.imath.kiev.ua/index.php/umj/article/view/5177.