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Computing Thiele Rules on Interval Elections and their Generalizations

Summary

This paper resolves the open question of computing Thiele rules on the voter interval domain by showing that the standard linear program admits an optimal integer solution, and provides a fast algorithm. It extends results to VCI and LC domains, proves LC strictly contains VCI, and shows that a tree-based generalization leads to NP-hardness.

SourcearXiv AIAuthor: Dimitris Avramidis, Alexandra Lassota, Ulrike Schmidt-Kraepelin, Adrian Vetta
Computing Thiele Rules on Interval Elections and their Generalizations
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[2605.03067] Computing Thiele Rules on Interval Elections and their Generalizations

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Computer Science > Artificial Intelligence

arXiv:2605.03067 (cs)

[Submitted on 4 May 2026]

Title:Computing Thiele Rules on Interval Elections and their Generalizations

Authors:Dimitris Avramidis, Alexandra Lassota, Ulrike Schmidt-Kraepelin, Adrian Vetta

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Abstract:Approval-based committee voting has received significant attention in the social choice community. Among the studied rules, Thiele rules, and especially Proportional Approval Voting (PAV), stand out for desirable properties such as proportional representation, Pareto optimality, and support monotonicity. Their main drawback is that computing a Thiele outcome is NP-hard in general.

A glimpse of hope comes from the fact that Thiele rules are better behaved under structured preferences. On the candidate interval (CI) domain, they are computable in polynomial time via a linear program (LP) that has a totally unimodular constraint matrix. Surprisingly, this approach fails for the related voter interval (VI) domain, and the complexity of the problem has repeatedly been posed as an open question. Our main result resolves this question: although the relevant matrix is not totally unimodular, the ``standard'' LP still admits at least one optimal integral solution, and we provide a fast algorithm for finding it.

Our technique naturally extends to the voter-candidate interval (VCI) domain, also known as the 1-dimensional voter-candidate range (1D-VCR) domain, and to the linearly consistent (LC) domain, both of which generalize the candidate and voter interval domains. Although both the VCI and LC domains have been studied in social choice, their relationship was unknown. We show, through connections to graph theory, that LC strictly contains VCI. We also provide an alternative definition of LC that is closer in spirit to VCI and has a natural interpretation in approval elections; this equivalence may be of independent interest. Finally, we study an alternative tree-based generalization of VCI and show that Thiele rules become NP-hard to compute on this domain.

Comments: 19 pages

Subjects:

Artificial Intelligence (cs.AI); Computer Science and Game Theory (cs.GT)

Cite as: arXiv:2605.03067 [cs.AI]

(or arXiv:2605.03067v1 [cs.AI] for this version)

https://doi.org/10.48550/arXiv.2605.03067

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arXiv-issued DOI via DataCite (pending registration)

Submission history

From: Adrian Vetta [view email] [v1] Mon, 4 May 2026 18:39:48 UTC (26 KB)

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Key points

  • Thiele rules like PAV are NP-hard in general but tractable under structured preferences.
  • Solves long-standing open problem for voter interval domain: standard LP has integer optimal solution.
  • Technique extends to voter-candidate interval and linearly consistent domains; LC strictly contains VCI.
  • Tree-based generalization makes Thiele rules NP-hard again, delineating tractability boundaries.

Highlights and analysis are generated automatically and may contain errors. Check the original source.