Khafagy, Salah. “Maximum Principle and Existence of Weak Solutions for Nonlinear System Involving Singular (p,q)-Laplacian Operators”. Journal of Progressive Research in Mathematics 7, no. 4 (June 6, 2016): 1153-1163. Accessed May 2, 2024. http://scitecresearch.com/journals/index.php/jprm/article/view/773.