Kailash C. Madan,  On Four Models of a Three Server Queue with Optional Server Vacations.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    July 2016, vol. 116(8). 400 - 404
    [viewed 28 January 2021]. Available from: https://publications.waset.org/pdf/10005293.