Berezansky, Yu. M., et al. “Eigen-Functions Decomposition of Commutated operators’ Families and Representations of Commutational Relations ”. Ukrains’kyi Matematychnyi Zhurnal, vol. 40, no. 1, Dec. 1987, pp. 106-9, https://umj.imath.kiev.ua/index.php/umj/article/view/9064.