Ostrovskii, V. L., and Yu. S. Samoilenko. “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.