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Manipulation Complexity of Same-System Runoff Elections

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Abstract

Do runoff elections, using the same voting rule as the initial election but just on the winning candidates, increase or decrease the complexity of manipulation? Does allowing revoting in the runoff increase or decrease the complexity relative to just having a runoff without revoting? For both weighted and unweighted voting, we show that even for election systems with simple winner problems the complexity of manipulation, manipulation with runoffs, and manipulation with revoting runoffs are independent. On the other hand, for some important, well-known election systems we determine what holds for each of these cases. For no such systems do we find runoffs lowering complexity, and for some we find that runoffs raise complexity. Ours is the first paper to show that for natural, unweighted election systems, runoffs can increase the manipulation complexity.

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Authors and Affiliations

  1. College of Computing and Information Sciences, Rochester Institute of Technology, Rochester, NY, 14623, USA

    Zack Fitzsimmons

  2. Department of Computer Science, Rochester Institute of Technology, Rochester, NY, 14623, USA

    Edith Hemaspaandra

  3. Department of Computer Science, University of Rochester, Rochester, NY, 14627, USA

    Lane A. Hemaspaandra

Authors
  1. Zack Fitzsimmons
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  2. Edith Hemaspaandra
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  3. Lane A. Hemaspaandra
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Correspondence to Lane A. Hemaspaandra.

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Fitzsimmons, Z., Hemaspaandra, E. & Hemaspaandra, L.A. Manipulation Complexity of Same-System Runoff Elections. Ann Math Artif Intell 77, 159–189 (2016). https://doi.org/10.1007/s10472-015-9490-6

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