BUGIR, M. K. Study of properties of firmness of solutions of equations with partial derivatives by the method of separation of variables . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 44, n. 3, p. 317–323, 1992. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8100. Acesso em: 5 dec. 2025.