Two-Price Equilibrium

Michal Feldman, Galia Shabtai, Aner Wolfenfeld

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review


Walrasian equilibrium is a prominent market equilibrium notion, but rarely exists in markets with indivisible items. We introduce a new market equilibrium notion, called two-price equilibrium (2PE). A 2PE is a relaxation of Walrasian equilibrium, where instead of a single price per item, every item has two prices: one for the item's owner and a (possibly) higher one for all other buyers. Thus, a 2PE is given by a tuple (S, p, p) of an allocation S and two price vectors p, p, where every buyer i is maximally happy with her bundle Si, given prices p for items in Si and prices p for all other items. 2PE generalizes previous market equilibrium notions, such as conditional equilibrium, and is related to relaxed equilibrium notions like endowment equilibrium. We define the discrepancy of a 2PE - a measure of distance from Walrasian equilibrium - as the sum of differences pj−pj over all items (normalized by social welfare). We show that the social welfare degrades gracefully with the discrepancy; namely, the social welfare of a 2PE with discrepancy d is at least a fraction (Equation presented) of the optimal welfare. We use this to establish welfare guarantees for markets with subadditive valuations over identical items. In particular, we show that every such market admits a 2PE with at least 1/7 of the optimal welfare. This is in contrast to Walrasian equilibrium or conditional equilibrium which may not even exist. Our techniques provide new insights regarding valuation functions over identical items, which we also use to characterize instances that admit a WE.

Original languageEnglish
Title of host publicationAAAI-22 Technical Tracks 5
PublisherAssociation for the Advancement of Artificial Intelligence
Number of pages8
ISBN (Electronic)1577358767, 9781577358763
StatePublished - 30 Jun 2022
Event36th AAAI Conference on Artificial Intelligence, AAAI 2022 - Virtual, Online
Duration: 22 Feb 20221 Mar 2022

Publication series

NameProceedings of the 36th AAAI Conference on Artificial Intelligence, AAAI 2022


Conference36th AAAI Conference on Artificial Intelligence, AAAI 2022
CityVirtual, Online


Dive into the research topics of 'Two-Price Equilibrium'. Together they form a unique fingerprint.

Cite this