Exact analysis of Dodgson elections: Lewis Carroll's 1876 voting system is complete for parallel access to NP

  • Edith Hemaspaandra
  • Lane A. Hemaspaandra
  • Jörg Rothe
Session 3: Computational Complexity
Part of the Lecture Notes in Computer Science book series (LNCS, volume 1256)


In 1876, Lewis Carroll proposed a voting system in which the winner is the candidate who with the fewest changes in voters' preferences becomes a Condorcet winner— a candidate who beats all other candidates in pairwise majority-rule elections. Bartholdi, Tovey, and Trick provided a lower bound— NP-hardness — on the computational complexity of determining the election winner in Carroll's system. We provide a stronger lower bound and an upper bound that matches our lower bound. In particular, determining the winner in Carroll's system is complete for parallel access to NP, i.e., it is complete for Θ 2 p , for which it becomes the most natural complete problem known. It follows that determining the winner in Carroll's elections is not NP-complete unless the polynomial hierarchy collapses.


Vote System Preference Order Condorcet Winner Preference List Polynomial Hierarchy 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© Springer-Verlag Berlin Heidelberg 1997

Authors and Affiliations

  • Edith Hemaspaandra
    • 1
  • Lane A. Hemaspaandra
    • 2
  • Jörg Rothe
    • 3
  1. 1.Department of MathematicsLe Moyne CollegeSyracuseUSA
  2. 2.Department of Computer ScienceUniversity of RochesterRochesterUSA
  3. 3.Institut für InformatikFriedrich-Schiller-Universität JenaJenaGermany

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