GORBACHUK, M. L.; HRUSHKA, Ya. I.; TORBA, S. M. Direct and Inverse Theorems in the Theory of Approximation by the Ritz Method. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 57, n. 5, p. 633–643, 2005. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3629. Acesso em: 5 dec. 2025.