Ant colony systems for large sequential ordering problems

The sequential ordering problem is a version of the asymmetric traveling salesman problem where precedence constraints on vertices are imposed. A tour is feasible if these constraints are respected, and the objective is to find a feasible solution with minimum cost. The sequential ordering problem models a lot of real world applications, mainly in the fields of transportation and production planning. In this paper we propose an extension of a well known ant colony system for the problem, aiming at making the approach more efficient on large problems. The extension is based on a problem manipulation technique that heuristically reduces the search space. Computational results, where the extended ant colony system is compared to the original one, are presente