Orochko Yu. B. “Essential Self-Conjugacy of Operators Connected With the Cauchy, Problem for the Wave Equation ”. Ukrains’kyi Matematychnyi Zhurnal 44, no. 7 (August 3, 1992): 940-948. Accessed October 6, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/8127.