KUZHEL', S. A.; PATSYUK, O. M. On the theory of $\mathcal{PT}$-symmetric operatorss. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 64, n. 1, p. 32–49, 2012. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/2554. Acesso em: 5 dec. 2025.