VLASII, O. D.; PTASHNIK, B. I. Nonlocal boundary-value problem for linear partial differential equations unsolved with respect to the higher time derivative. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 59, n. 3, p. 370–381, 2007. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3313. Acesso em: 5 dec. 2025.