BratiichukM. S., Chechelnitsky A. A., and UsarI. Y. “M/M/1 Retrial Queueing Model With Variable Service Rate”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 72, no. 3, Mar. 2020, pp. 355-6, doi:10.37863/umzh.v72i3.813.