When are elections with few candidates hard to manipulate? (2007)
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| Venue: | JOURNAL OF THE ACM |
| Citations: | 156 - 18 self |
BibTeX
@ARTICLE{Conitzer07whenare,
author = {Vincent Conitzer and TUOMAS SANDHOLM and Jérôme Lang},
title = {When are elections with few candidates hard to manipulate?},
journal = {JOURNAL OF THE ACM},
year = {2007}
}
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Abstract
In multiagent settings where the agents have different preferences, preference aggregation is a central issue. Voting is a general method for preference aggregation, but seminal results have shown that all general voting protocols are manipulable. One could try to avoid manipulation by using protocols where determining a beneficial manipulation is hard. Especially among computational agents, it is reasonable to measure this hardness by computational complexity. Some earlier work has been done in this area, but it was assumed that the number of voters and candidates is unbounded. Such hardness results lose relevance when the number of candidates is small, because manipulation algorithms that are exponential only in the number of candidates (and only slightly so) might be available. We give such an algorithm for an individual agent to manipulate the Single Transferable Vote (STV) protocol, which has been shown hard to manipulate in the above sense. This motivates the core of this paper, which derives hardness results for realistic elections where the number of candidates is a small constant (but the number of voters can be large). The main manipulation question we study is that of coalitional manipulation by weighted voters. (We show that for simpler manipulation problems, manipulation cannot be hard with few candidates.) We study both constructive manipulation (making a given candidate win) and de-
Keyphrases
hardness result preference aggregation candidate win individual agent bene cial manipulation manipulation problem weighted vot-ers multiagent setting erent preference general voting protocol cen-tral issue single transferable vote manipulation algorithm small constant manipulation cannot general method realistic election coalitional manipulation computational complexity constructive manipulation computational agent seminal result main manipulation question
