Orochko , Y.B. (1992) “Essential self-conjugacy of operators connected with the Cauchy, problem for the wave equation ”, Ukrains’kyi Matematychnyi Zhurnal, 44(7), pp. 940–948. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/8127 (Accessed: 6 December 2025).