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Cox, D. R. (2008). "The analysis of non-Markovian stochastic processes by the inclusion of supplementary variables". Mathematical Proceedings of the Cambridge Philosophical Society. 51 (3): 433. doi:10.1017/S0305004100030437.
Yashkov, S. F. (2008). "A note on application of the method of supplementary variables to the analysis of a processor sharing system". Automation and Remote Control. 69 (9): 1622–1629. doi:10.1134/S0005117908090166.