Yusuf, Ibrahim. “Availability and Profit Analysis of 3-Out-of-4 Repairable System With Preventive Maintenance”. International Journal of Applied Mathematical Research 1, no. 4 (September 2, 2012): 510–519. Accessed May 2, 2024. https://www.sciencepubco.com/index.php/ijamr/article/view/345.