TSYNALKO, P. V. On a smooth solution of a boundary-value problem. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 50, n. 9, p. 1293–1296, 1998. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4849. Acesso em: 6 dec. 2025.