“…The first paper of this decade is difficult to identify as there are several papers that appeared around the same time. The combinatorial approach started by Dudek and Teuton (1964) was corrected and improved by Smith and Dudek (1967), McMahon (1969), Gupta (1971), and Szwarc (1971Szwarc ( , 1973. The branch and bound solution approaches were also developed during this time by Lomnicki (1965), Brown and Lomnicki (1966), McMahon and Burton (1967), Gupta (1969bGupta ( , 1970, and others.…”