15.6. Consider the following instance of a single machine with sequence dependent setup times. The objective to be minimized is the makespan. There are 6 jobs. The sequence dependent setup times are specified in the table below. Assume K to be very large. Define as the neighbourhood of a schedule all schedules that can be obtained through an adjacent pairwise interchange. (a) Find the optimal sequence. (b) Determine the makespans of all schedules that are neighbors of the optimal schedule. (c) Find a schedule, with a makespan less than K, of which all neighbors have the same makespan. (The optimal sequence may be described as a "brittle" sequence, while the last sequence may be described as a more "robust" sequence.)