Scheduling jobs of equal length: complexity, facets and computational results

Y. Crama, F.C.R. Spieksma

Research output: Contribution to journalArticleAcademicpeer-review

13 Citations (Scopus)


The following problem was originally motivated by a question arising in the automated assembly of printed circuit boards. Given aren jobs, which have to be performed on a single machine within a fixed timespan [0,T], subdivided intoT unit-length subperiods. The processing time (or length) of each job equalsp,p ∈ ℕ. The processing cost of each job is an arbitrary function of its start-time. The problem is to schedule all jobs so as to minimize the sum of the processing costs.

This problem is proved to be NP-hard, already forp=2 and 0–1 processing costs. On the other hand, whenT=np+c, withc constant, the problem can be solved in polynomial time. A partial polyhedral description of the set of feasible solutions is presented. In particular, two classes of facet-defining inequalities are described, for which the separation problem is polynomially solvable. Also, we exhibit a class of objective functions for which the inequalities in the LP-relaxation guarantee integral solutions.

Finally, we present a simple cutting plane algorithm and report on its performance on randomly generated problem instances.
Original languageEnglish
Pages (from-to)207-227
JournalMathematical Programming
Issue number3
Publication statusPublished - 31 Mar 1996
Externally publishedYes


  • scheduling
  • computational complexity
  • polyhedral description


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