TAILIEUCHUNG - Báo cáo "Algorithm for solution of a routing problem "

In this paper we consider a combinatorial optimization problem that is similar to the bottleneck traveling salesman problem. We show that an optimal tour for this problem is pyramidal tour (1, 3, 5, , n, , 6, 4, 2) or consists of some pyramidal subtours. The above 7methods can be extended to complete bipartite graphs. 1. Problem statement It is well-known that the traveling salesman problem (TSP) is strongly NP-hard (cf. [1], p. 353). But for some special cases of the TSP can be solvable in polynomial time. . | VNU Journal of Science Mathematics - Physics 23 2007 178-182 Algorithm for solution of a routing problem Tran Vu Thieu Pham Xuan Hinh Institute of Mathematics 18 Hoang Quoc Viet Cau Giay Hanoi Vietnam Received 15 November 2006 received in revised form 12 September 2007 Abstract. In this paper we consider a combinatorial optimization problem that is similar to the bottleneck traveling salesman problem. We show that an optimal tour for this problem is pyramidal tour 1 3 5 . n. 6 4 2 or consists of some pyramidal subtours. The above 7methods can be extended to complete bipartite graphs. 1. Problem statement It is well-known that the traveling salesman problem TSP is strongly NP-hard cf. 1 p. 353 . But for some special cases of the TSP can be solvable in polynomial time. This is the case where the distance matrix in the TSP fulfills certain additional conditions . the Monge property Kalmanson matrices the Demidenko conditions or the Supnick conditions cf. 2-4 . In the sequel we will introduce another special case of the TSP which can easily be solvable and show that the optimal tour for this problem is pyramidal tour or consists of some at most three pyramidal subtours. Consider a complete graph G A E with vertex set A a1 a2 . an and edge set E Ax A. Each vertex ai e A has a real number ti i 1 . n called the altitude of vertex ai. We specify vertices ab e A the source and ae e A the sink such that b e e 1 2 . n and tb te. Consider the following problem called Problem A for short Problem A. Find a Hamiltonian path in the graph from ab to ae which visits every vertex exactly once so that to minimize the highest difference between altitudes of any two successive vertices in the path. In other word among permutations n i1 i2 . in of the numbers 1 2 . n with i1 b in e find a permutation so that to minimize the function f n max t. -1. min. 1 k n-1 I k 1 1 I It is easy to see that such a permutation corresponds to a Hamiltonian path in the graph from ab to ae and that the

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