E.M. Badr, and M.I. Moussa. “On Jump-Critical Ordered Sets With Jump Number Four”. Journal of Advances in Applied & Computational Mathematics 1, no. 1 (October 10, 2014): 8–13. Accessed July 3, 2022. https://www.avantipublishers.com/index.php/jaacm/article/view/55.