KOCHMANSKI, S. On the evolution operators for some equations of mathematical physics with variable coefficients. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 46, n. 7, p. 857–869, 1994. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5677. Acesso em: 6 dec. 2025.