Thomas A. Ryan Jr. Edward G. Coffman Jr. 1116-1122 1974 23 IEEE Trans. Computers 11 https://doi.org/10.1109/T-C.1974.223818 http://doi.ieeecomputersociety.org/10.1109/T-C.1974.223818 db/journals/tc/tc23.html#RyanC74