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We prove that the problem of sorting a permutation by the minimum number of reversals is NP-hard, thus answering a major question on the complexity of a problem which has widely been studied in the last years. The proof is based on the strong relationship between this problem and the problem of finding the maximum number of edge-disjoint alternating cycles… (More)

We present a Lagrangian-based heuristic for the well-known Set Covering Problem (SCP). The algorithm was initially designed for solving very large scale SCP instances, involving up to 5,000 rows and 1,000,000 columns, arising from crew scheduling in the Italian Railway Company, Ferrovie dello Stato SpA. In 1994 Ferrovie dello Stato SpA, jointly with the… (More)

Given the integer polyhedron P I := convfx 2 Z n : Ax bg, w h e r e A 2 Z mn and b 2 Z m , a Chvv atal-Gomory (CG) cut is a valid inequality f o r P I of the type T Ax b T bc for some 2 R m + such that T A 2 Z n. In this paper we study f0 1 2 g-CG cuts, arising for 2 f 0 1=2g m. W e show that the associated separation problem, f0 1 2 g-SEP, i s e q u i v… (More)

The Set Covering Problem (SCP) is a main model for several important applications, including crew scheduling in railway and mass-transit companies. In this survey, w e focus our attention on the most recent and eeective algorithms for SCP, considering both heuristic and exact approaches, outlining their main characteristics and presenting an experimental… (More)

We analyze the strong relationship among three combinatorial problems, namely, the problem of sorting a permutation by the minimum number of reversals (MIN-SBR), the problem of finding the maximum number of edge-disjoint alternating cycles in a breakpoint graph associated with a given permutation (MAX-ACD), and the problem of partitioning the edge set of an… (More)

The Quadratic Knapsack Problem (QKP) calls for maximizing a quadratic objective function subject to a knapsack constraint, where all coeecients are assumed to be nonnegative and all variables are binary. The problem has applications in location and hydrology, and generalizes the problem of checking whether a graph contains a clique of a given size. We… (More)