OstrovskiiV. L., and SamoilenkoY. S. “On Pairs of Unbounded Self-Adjoint Operators Satisfying an Algebraic Relation”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 45, no. 9, Sept. 1993, pp. 1253–1258, https://umj.imath.kiev.ua/index.php/umj/article/view/5927.