Kosyak, O. V., and L. P. Nizhnik. “Operators of Generalized Translation and Hypergroups Constructed from Self-Adjoint Differential Operators”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, no. 5, May 2005, pp. 659–668, https://umj.imath.kiev.ua/index.php/umj/article/view/3632.