Papers by François Soumis
Transportation Research Part B: Methodological, 2006
This paper considers the locomotive assignment problem encountered during the planning of the ope... more This paper considers the locomotive assignment problem encountered during the planning of the operations of a freight railroad, which consists of providing sufficient motive power to pull a set of scheduled trains at minimum cost while satisfying locomotive availability and maintenance requirements. In 1997, Ziarati et al. proposed for this problem a heuristic branch-and-price approach that relies on a simple depth-first search strategy without backtracking. In this paper, we present an efficient backtracking mechanism that can be added to this heuristic branch-and-price approach. To do so, we propose and evaluate different branching methods that impose multiple decisions on locomotive routes at each branching node, including one decision that forbids one such route. Finally, we introduce different ways of computing an estimate of the best integer solution value that can be obtained from a branch-and-bound node. These estimates can be used to guide the backtracking process of a two-phase search strategy.
ABSTRACT In this article, we propose a general framework for an algorithm derived from the primal... more ABSTRACT In this article, we propose a general framework for an algorithm derived from the primal simplex that guarantees a strict improvement in the objective after each iteration. Our approach relies on the identification of compatible variables that ensure a nondegenerate iteration if pivoted into the basis. The problem of finding a strict improvement in the objective function is proved to be equivalent to two smaller problems respectively focusing on compatible and incompatible variables. We then show that the improved primal simplex (IPS) of Elhallaoui et al. is a particular implementation of this generic theoretical framework. The resulting new description of IPS naturally emphasizes what should be considered as necessary adaptations of the framework versus specific implementation choices. This provides original insight into IPS that allows for the identification of weaknesses and potential alternative choices that would extend the e�ciency of the method to a wider set of problems. We perform experimental tests on an extended collection of data sets including instances of Mittelmann’s benchmark for linear programming. The results confirm the excellent potential of IPS and highlight some of its limits while showing a path toward an improved implementation of the generic algorithm.
Given a set of scheduled flights that must be operated by the same aircraft type, the aircraft ro... more Given a set of scheduled flights that must be operated by the same aircraft type, the aircraft routing problem consists of building anonymous aircraft routes that respect maintenance requirements and cover each flight exactly once. This paper looks at the nature of the problem and introduces a classification according to three business processes that are used to assign the anonymous routes to the specific aircraft tail numbers. Furthermore, we compare the aircraft routing problem variants resulting from these three processes with regard to their adaptability to different contexts, the difficulty of solving them, the cost of the computed solutions, and the robustness of these solutions.
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Papers by François Soumis