KOMARNYTS'KA, L. I.; PTASHNIK, B. I. Boundary-value problems for differential equations unsolvable with respect to the higher time derivative. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 47, n. 9, p. 1197–1208, 1995. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5519. Acesso em: 6 dec. 2025.