L.A. Wolsey

Publications (6)3.41 Total impact

• Article: Scheduling Projects with Labour Constraints

  C. C. B. Cavalcante, C. Carvalho De Souza, M. W. P. Savelsbergh, Y. Wang, L.A. Wolsey
  [show abstract] [hide abstract]
  ABSTRACT: In this paper we consider a labour constrained scheduling problem (LCSP) which is a simplification of a practical problem arising in industry. Jobs are subject to precedence constraints and have specified processing times. Moreover, for each job the labour requirement varies as the job is processed. Given the amount of labour available in each period, the problem is to finish all the jobs as soon as possible, that is, to minimize makespan, subject to the precedence and labour constraints. Several Integer Programming (IP) formulations for this problem are discussed and valid inequalities for these different models are introduced. It turns out that a major drawback in using the IP approach is the weakness of the lower bound relaxations. However, we report computational experiments showing how the solution of the linear relaxation of the IP models can be used to provide good schedules. Solutions arising from these LP-based heuristics are considerably improved by local search procedures. W...
  10/1998;
• Source

  Available from: Martin Savelsbergh

  Article: Scheduling Projects with Labor Constraints.

  C. C. B. Cavalcante, C. C. De Souza, M. W. P. Savelsbergh, Y. Wang, L.A. Wolsey
  [show abstract] [hide abstract]
  ABSTRACT: In this paper we consider a labor constrained scheduling problem (LCSP) which is a simplification of a practical problem arising in industry. Jobs are subject to precedence constraints and have specified processing times. Moreover, for each job the labor requirements varies as the job is processed. Given the amount of labor available in each period, the problem is to finish all the jobs as soon as possible, that is, to minimize makespan, subject to the precedence and labor constraints. Several Integer Programming (IP) formulations for this problem are discussed and valid inequalities for these different models are introduced.
  02/1998;
• Source

  Available from: psu.edu

  Article: The node capacitated graph partitioning problem: A computational study

  C. E. Ferreira, A. Martin, C. C. de Souza, R. Weismantel, L. A. Wolsey
  [show abstract] [hide abstract]
  ABSTRACT: In this paper we consider the problem ofk-partitioning the nodes of a graph with capacity restrictions on the sum of the node weights in each subset of the partition, and the objective of minimizing the sum of the costs of the edges between the subsets of the partition. Based on a study of valid inequalities, we present a variety of separation heuristics for cycle, cycle with ears, knapsack tree and path-block cycle inequalities among others. The separation heuristics, plus primal heuristics, have been implemented in a branch-and-cut routine using a formulation including variables for the edges with nonzero costs and node partition variables. Results are presented for three classes of problems: equipartitioning problems arising in finite element methods and partitioning problems associated with electronic circuit layout and compiler design. 1998 The Mathematical Programming Society, Inc. Published by Elsevier Science B.V.
  Mathematical Programming 01/1998; 81(2):229-256. · 1.71 Impact Factor
• Article: Formulations and valid inequalities for the node capacitated graph partitioning problem

  C. E. Ferreira, A. Martin, C. C. de Souza, R. Weismantel, L. A. Wolsey
  [show abstract] [hide abstract]
  ABSTRACT: We investigate the problem of partitioning the nodes of a graph under capacity restriction on the sum of the node weights in each subset of the partition. The objective is to minimize the sum of the costs of the edges between the subsets of the partition. This problem has a variety of applications, for instance in the design of electronic circuits and devices. We present alternative integer programming formulations for this problem and discuss the links between these formulations. Having chosen to work in the space of edges of the multicut, we investigate the convex hull of incidence vectors of feasible multicuts. In particular, several classes of inequalities are introduced, and their strength and robustness are analyzed as various problem parameters change.
  Mathematical Programming 01/1996; 74(3):247-266. · 1.71 Impact Factor
• Article: A New Approach to Minimising the Frontwidth in Finite Element Calculations.

  C. C. De Souza, R. Keunings, L.A. Wolsey, O. Zone
  02/1992;
• Article: Scheduling projects with labor constraints

  C.C.B. Cavalcante, C. Carvalho de Souza, M.W.P. Savelsbergh, Y. Wang, L.A. Wolsey
  [show abstract] [hide abstract]
  ABSTRACT: In this paper we consider a labor constrained scheduling problem (LCSP) which is a simplification of a practical problem arising in industry. Jobs are subject to precedence constraints and have specified processing times. Moreover, for each job the labor requirement varies as the job is processed. Given the amount of labor available in each period, the problem is to finish all the jobs as soon as possible, that is, to minimize the makespan, subject to the precedence and labor constraints. Several integer programming (IP) formulations for this problem are discussed and valid inequalities for these different models are introduced. It turns out that a major drawback in using an IP approach is the weakness of the lower bound relaxations. However, we report computational experiments showing how the solution of the linear relaxation of the IP models can be used to provide good schedules. Solutions arising from these LP-based heuristics are considerably improved by local search procedures. We further exploit the capabilities of local search for LCSP by designing a Tabu search algorithm. The computational experiments on a benchmark data set show that the Tabu search algorithm generates the best-known upper bounds for almost all these instances. We also show how IP can be used to provide reasonably good lower bounds for LCSP when the makespan is replaced by suitably modified objective functions. Finally, some directions for further investigations which may turn IP techniques into a more interesting tool for solving such a problem are suggested.
  Discrete Applied Mathematics.