Abstract
In the paper “The complexity of mean flow time scheduling problems with release times”, by Baptiste, Brucker, Chrobak, Dürr, Kravchenko and Sourd, the authors claimed to prove strong \({\mathbb {NP}}\)-hardness of the scheduling problem \(P|{\textit{pmtn}},r_j|\sum C_j\), namely multiprocessor preemptive scheduling where the objective is to minimize the mean flow time. We point out a serious error in their proof and give a new proof of strong \({\mathbb {NP}}\)-hardness for this problem.



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Acknowledgments
M. Chrobak was partially supported by National Science Foundation Grant CCF-1217314. The authors would like to thank anonymous reviewers for many useful comments and suggestions.
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Bellenguez-Morineau, O., Chrobak, M., Dürr, C. et al. A note on \({\mathbb {NP}}\)-hardness of preemptive mean flow-time scheduling for parallel machines. J Sched 18, 299–304 (2015). https://doi.org/10.1007/s10951-014-0380-2
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DOI: https://doi.org/10.1007/s10951-014-0380-2