In this article we answer the complexity question of two dual criteria scheduling problems which had been open for a long time. Both problems are single machine scheduling problems and both have the number of tardy jobs as the primary criterion. However, the first problem has the total completion time and the second one has the total tardiness as the secondary criterion. We show that both problems are binary NP-hard.
Yumei Huo, Joseph Y.-T. Leung, Hairong Zhao