Mel’nichenko, I. P. “Algebras of Functionally Invariant Solutions of the Three-Dimensional Laplace Equation”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 55, no. 9, Sept. 2003, pp. 1284-90, https://umj.imath.kiev.ua/index.php/umj/article/view/4002.