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 61, no. 4 (December 30, 2016): 443–451. Accessed February 5, 2025. https://studia.reviste.ubbcluj.ro/index.php/subbmathematica/article/view/5636.