PRECUM R. 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. Stud. Univ. Babeş-Bolyai Math. [Internet]. 2016 Dec. 30 [cited 2025 Apr. 19];61(4):443-51. Available from: https://studia.reviste.ubbcluj.ro/index.php/subbmathematica/article/view/5636