A new perspective on implementation by voting trees

Felix Fischer, Ariel D. Procaccia*, Alex Samorodnitsky

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

8 Scopus citations


Voting trees describe an iterative procedure for selecting a single vertex from a tournament. They provide a very general abstract model of decision-making among a group of individuals, and it has therefore been studied which voting rules have a tree that implements them, i.e., chooses according to the rule for every tournament. While partial results concerning implementable rules and necessary conditions for implementability have been obtained over the past 40 years, a complete characterization of voting rules implementable by trees has proven surprisingly hard to find. A prominent rule that cannot be implemented by trees is the Copeland rule, which singles out vertices with maximum degree. In this paper, we suggest a new angle of attack and re-examine the implementability of the Copeland solution using paradigms and techniques that are at the core of theoretical computer science. We study the extent to which voting trees can approximate the maximum degree in a tournament, and give upper and lower bounds on the worst-case ratio between the degree of the vertex chosen by a tree and the maximum degree, both for the deterministic model concerned with a single fixed tree, and for randomizations over arbitrary sets of trees. Our main positive result is a randomization over surjective trees of polynomial size that provides an approximation ratio of at least 1/2. The proof is based on a connection between a randomization over caterpillar trees and a rapidly mixing Markov chain.

Original languageAmerican English
Pages (from-to)59-82
Number of pages24
JournalRandom Structures and Algorithms
Issue number1
StatePublished - Aug 2011


  • Approximation
  • Markov chains
  • Social choice


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