M. A. Ferreira,  "Mean sojourn time in state k, k=0,1,..., for the M|G|∞ queueing system (Exact and approximated expressions)", in Proc. of the Int. Conf. Quantitative Methods in Economics (Multiple Criteria Decision Making XI), Magáthová, V., Ed., Nitra, Slovakia, Slovak Society for Operations Research, 2002, pp. 57-61