Li, W., Alfa, A.S. and Zhao, Y.Q. (l998) Stochastic Analysis of a Repairable System with Three Units and Two Repair Facilities. Microelectronics and Reliability, 38, 585-595.
 Kovalenko, A.I. (2001) Analysis of the Reliability of a Three-Component System with Renewal. Journal of Mathematical Science, 103, 273-277.
 Hu, L., Tian, R., Wu, J. and Cao, J. (2007) Reliability Analysis of Series’-Parallel Repairable System Consisting of Three Units with Vacation. Journal of Yanshan University, 103, 299-303.
 Haji, A., Keyim, T. and Yunus, B. (2016) Existence and Uniqueness of the Dynamic Solution of a Series-Parallel Repairable System Consisting of Three-Unit with Multiple Vacations of a Repairman. Journal of Applied Mathematics and Physics, 4, 1585-1591.
 Engel, K.-J. and Nagel, R. (2000) One-Parameter Semigroups for Linear Evolution Equations. Graduate Texts in Mathematics, Springer-Verlag, Berlin, 194.
 Nagel, R. (1986) One-Parameter Semigroups of Positive Operators. Springer-Verlag, Berlin.
 Haji, A. and Radl, A. (2007) A Semigroup Approach to Queueing Systems. Semigroup Forum, 75, 609-623.