OROCHKO , Yu. B. Essential self-conjugacy of operators connected with the Cauchy, problem for the wave equation . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 44, n. 7, p. 940–948, 1992. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8127. Acesso em: 6 dec. 2025.