PRECUM, Radu. Abstract method of upper and lower solutions and application to singular boundary value problems: Dedicated to Professor Ioan A. Rus on the occasion of his 80th anniversary. Studia Universitatis Babeș-Bolyai Mathematica, [S. l.], v. 61, n. 4, p. 443–451, 2016. Disponível em: https://studia.reviste.ubbcluj.ro/index.php/subbmathematica/article/view/5636. Acesso em: 5 feb. 2025.