论文标题

对社会选择的平滑分析,重新审视

Smoothed Analysis of Social Choice, Revisited

论文作者

Flanigan, Bailey, Halpern, Daniel, Psomas, Alexandros

论文摘要

社会选择中的一个规范问题是如何汇总排名票:鉴于$ n $选民在$ M $候选人上的排名,我们应该使用哪些投票规则$ f $将这些投票汇总到单个赢家中?比较投票规则的一种标准方法是对公理的满意度 - 我们希望“合理”规则满足的属性。不幸的是,这种方法导致了几种不可能:没有投票规则可以同时满足我们想要的所有属性,至少在所有可能的输入中最坏的情况下。在此激励的情况下,我们考虑放松这些最坏情况的要求。我们使用一种“平滑”的社会选择模型这样做,在这种模型中,投票与少量的噪音扰动。如果我们从哪个输入配置文件开始,那么满足公理的概率(噪声)很大,我们将认为该公理与满足的公理一样好 - 即使在最坏的情况下可能会违反它的“平滑满足感”。我们的模型是对Lirong Xia的限制,与Spielman和Teng在平滑分析上的原始工作紧密相对应。迄今为止,Xia的几篇论文已经完成了许多工作,以在这种噪声下对公理满意度进行。在我们的论文中,我们的目标是对何时平滑的社会选择分析有用,以更加凝聚力。在我们的模型中,我们提供了简单的条件,以使几个先前未研究的公理和悖论的平滑满足或平滑侵入,以及许多由XIA研究的悖论。然后,我们观察到,在实际上重要的噪声模型子类中,尽管融合最终发生,但已知的速率可能需要大量的选民。在此激励的情况下,我们在此子类-Mallows模型的规范噪声模型中特别证明了界限。在这里,我们对何时平滑分析可以提供更细微的图片。

A canonical problem in social choice is how to aggregate ranked votes: given $n$ voters' rankings over $m$ candidates, what voting rule $f$ should we use to aggregate these votes into a single winner? One standard method for comparing voting rules is by their satisfaction of axioms - properties that we want a "reasonable" rule to satisfy. Unfortunately, this approach leads to several impossibilities: no voting rule can simultaneously satisfy all the properties we want, at least in the worst case over all possible inputs. Motivated by this, we consider a relaxation of these worst case requirements. We do so using a "smoothed" model of social choice, where votes are perturbed with small amounts of noise. If, no matter which input profile we start with, the probability (post-noise) of an axiom being satisfied is large, we will consider the axiom as good as satisfied - called "smoothed-satisfied" - even if it may be violated in the worst case. Our model is a mild restriction of Lirong Xia's, and corresponds closely to that in Spielman and Teng's original work on smoothed analysis. Much work has been done so far in several papers by Xia on axiom satisfaction under such noise. In our paper, we aim to give a more cohesive overview on when smoothed analysis of social choice is useful. Within our model, we give simple sufficient conditions for smoothed-satisfaction or smoothed-violation of several previously-unstudied axioms and paradoxes, plus many of those studied by Xia. We then observe that, in a practically important subclass of noise models, although convergence eventually occurs, known rates may require an extremely large number of voters. Motivated by this, we prove bounds specifically within a canonical noise model from this subclass - the Mallows model. Here, we present a more nuanced picture on exactly when smoothed analysis can help.

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