On the existence of dominating 6-cyclic schedules in four-machine robotic cells

We consider a four-machine robotic cell producing identical parts and served by a single robot. We study the no-wait multi-cyclic scheduling problem. Using the forbidden-intervals method, we show that in such a cell the optimal schedule can be k-cyclic with minimum k ≥ 6. This fact refutes Agnetis' conjecture (Agnetis, 2000) stating that the minimum k for the optimal k-cyclic m-machine schedules does not exceed m−1. In particular, we construct a counter-example to Agnetis' conjecture.