Latif, Abdul, and Saleh A. Al-Mezel. “Coincidence and fixed Point Results for Non-Commuting Maps”. Tamkang Journal of Mathematics 39, no. 2 (June 30, 2008): 105–110. Accessed July 6, 2022. https://journals.math.tku.edu.tw/index.php/TKJM/article/view/20.