Ostrovsky , V. L., and Yu. S. Samoilenko. “Application of the Projectional Spectral Theorem to Non-Commutating Operators Families”. Ukrains’kyi Matematychnyi Zhurnal, vol. 40, no. 4, June 1988, pp. 469-81, https://umj.imath.kiev.ua/index.php/umj/article/view/9182.