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[Wag87] Klaus W. Wagner. More complicated questions about maxima and minima, and some closures of NP. Theoretical Computer Science 51:53-80. Elsevier, 1987. 
@article{tcs51()-Wag,
  author =              {Wagner, Klaus W.},
  title =               {More complicated questions about maxima and minima,
                         and some closures of~{NP}},
  publisher =           {Elsevier},
  journal =             {Theoretical Computer Science},
  volume =              {51},
  pages =               {53-80},
  year =                {1987},
}
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