Exact Analysis of Dodgson Elections: Lewis Carroll's 1876 Voting System is Complete for Parallel Access to NP

TL;DR

This paper proves that determining the winner in Lewis Carroll's 1876 voting system is complete for parallel access to NP, establishing its precise computational complexity and implications for the polynomial hierarchy.

Contribution

The paper establishes that the winner determination problem in Carroll's voting system is complete for , matching bounds and refining previous NP-hardness results.

Findings

Winner determination is -complete.

The problem is not NP-complete unless the polynomial hierarchy collapses.

Provides tight bounds on the computational complexity of Carroll's voting system.

Abstract

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 $\thetatwo$, 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.