Bolch, Gunter; Greiner, Stefan; de Meer, Hermann; Trivedi, Kishor S. (2006). Queuing Networks and Markov Chains: Modeling and Performance Evaluation with Computer Science Applications (2nd ed.). Wiley. pp. 24–25. doi:10.1002/0471791571. ISBN978-0-471-79157-7.
Amari, Suprasad V.; Misra, Ravindra B. (1997). "Closed-form expressions for distribution of sum of exponential random variables". IEEE Transactions on Reliability. 46 (4): 519–522. doi:10.1109/24.693785.