BlackmoreD., PrykarpatskyA. K., PrykarpatskyY. A., and SamoilenkoA. M. “Theory of Multidimensional Delsarte – Lions Transmutation Operators. II”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 71, no. 6, June 2019, pp. 808-39, http://umj.imath.kiev.ua/index.php/umj/article/view/1478.