Technical Note—Johnson's Three-Machine Flow-Shop Conjecture

In an epoch-setting paper on scheduling theory, Johnson determined an efficient algorithm for solving the two-stage flow-shop problem and two special cases of the three-stage problem. In his concluding remarks he suggested another special condition in which one would obtain a three-stage solution. We give an example to show that this conjecture is not quite true. A true statement is obtained, however, by only a slight strengthening of the original. The primary issue concerns the impact of the two-stage no-preference precedent rules on the three-stage problem.