BOGDANSKY, Yu. V. The maximum principle for irregular elliptic differential equation in the countable-dimensional Hilbert space . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 40, n. 1, p. 21–25, 1987. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/9048. Acesso em: 6 dec. 2025.