# Stability of an optimal schedule for a flow-shop problem with two jobs

The problem F|n=2|F is to minimize the given objective function F(C1,m, C2,m) of completion times Ci,m of two jobs i Î J={1, 2} processed on m machines M={1, 2, …, m}. Both jobs have the same technological route through m machines. Processing time ti,k of job iÎ J on machine kÎ M is known. Operation preemptions are not allowed. Let R2m be space of non-negative 2m-dimensional real vectors t=(t1,1,…, t1,m, t2,1,…, t2,m) with Chebyshev’s distance d(t, t*). To solve problem F|n=2|F, we can use the geometric algorithm, which includes the following steps: 1) construct digraph (V, A) for problem F|n=2|F and find so-called border vertices in (V, A); 2) construct the set of trajectories corresponding to the shortest paths Rt in digraph (V, A) from the origin vertex to each of the border vertices; 3) find an optimal path in the set Rt that represents a schedule with minimal value of the objective function F. Let path tu Î Rt be optimal for the problem F|n=2|F with operation processing times defined by vector t. If for any small positive real number e > 0 there exists vector t*Î R2m such that d(t, t*) = e and path tu is not optimal for the problem F|n=2|F with operation processing times defined by vector t*, then optimality of path tu is not stable. The main result of the paper is the proof of necessary and sufficient conditions for optimality stability of path tu. If objective function F is continuous non-decreasing (e.g., makespan, total completion time, maximal lateness or total tardiness), then to test whether optimality of the path tu Î Rt is stable takes O(m log m) time.

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