VLASII, O. D.; PTASHNIK, B. I. A Problem with Nonlocal Conditions for Partial Differential Equations with Variable Coefficients. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 53, n. 10, p. 1328–1336, 2001. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4352. Acesso em: 6 dec. 2025.