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**1 - 1**of**1**### Equilibria Under the Probabilistic Serial Rule

"... The probabilistic serial (PS) rule is a prominent randomized rule for assigning indivisible goods to agents. Although it is well known for its good fair-ness and welfare properties, it is not strategyproof. In view of this, we address several fundamental questions regarding equilibria under PS. Firs ..."

Abstract
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The probabilistic serial (PS) rule is a prominent randomized rule for assigning indivisible goods to agents. Although it is well known for its good fair-ness and welfare properties, it is not strategyproof. In view of this, we address several fundamental questions regarding equilibria under PS. Firstly, we show that Nash deviations under the PS rule can cycle. Despite the possibilities of cycles, we prove that a pure Nash equilibrium is guaranteed to ex-ist under the PS rule. We then show that verify-ing whether a given profile is a pure Nash equi-librium is coNP-complete, and computing a pure Nash equilibrium is NP-hard. For two agents, we present a linear-time algorithm to compute a pure Nash equilibrium which yields the same assign-ment as the truthful profile. Finally, we conduct ex-periments to evaluate the quality of the equilibria that exist under the PS rule, finding that the vast majority of pure Nash equilibria yield social wel-fare that is at least that of the truthful profile. 1