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New Approximation Bounds for Lpt Scheduling

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We provide new bounds for the worst case approximation ratio of the classic Longest Processing Time (Lpt) heuristic for related machine scheduling (Q||C max ). For different machine speeds, Lpt was first considered by Gonzalez et al. (SIAM J. Comput. 6(1):155–166, 1977). The best previously known bounds originate from more than 20 years back: Dobson (SIAM J. Comput. 13(4):705–716, 1984), and independently Friesen (SIAM J. Comput. 16(3):554–560, 1987) showed that the worst case ratio of Lpt is in the interval (1.512,1.583), and in (1.52,1.67), respectively. We tighten the upper bound to \(1+\sqrt{3}/3\approx1.5773\) , and the lower bound to 1.54. Although this improvement might seem minor, we consider the structure of potential lower bound instances more systematically than former works. We present a scheme for a job-exchanging process, which, repeated any number of times, gradually increases the lower bound. For the new upper bound, this systematic method together with a new idea of introducing fractional jobs, facilitated a proof that is surprisingly simple, relative to the result. We present the upper-bound proof in parameterized terms, which leaves room for further improvements.

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Correspondence to Annamária Kovács.

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Kovács, A. New Approximation Bounds for Lpt Scheduling. Algorithmica 57, 413–433 (2010). https://doi.org/10.1007/s00453-008-9224-9

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  • Approximation algorithms
  • Related machine scheduling
  • LPT