Fair-Share Allocations for Agents with Arbitrary Entitlements

Abstract

We consider the problem of fair allocation of indivisible goods to n agents, with no transfers. When agents have equal entitlements, the well established notion of the maximin share (MMS) serves as an attractive fairness criterion, where to qualify as fair, an allocation needs to give every agent at least a substantial fraction of her MMS. In this paper we consider the case of arbitrary (unequal) entitlements. We explain shortcomings in previous attempts that extend the MMS to unequal entitlements. Our conceptual contribution is the introduction of a new notion of a share, the AnyPrice share (APS), that is appropriate for settings with arbitrary entitlements. The AnyPrice share of an agent is the value she can guarantee to herself if she is given a budget equal to her entitlement, and she buys her highest value affordable set when items are adversarially priced with a total price equal to the total entitlements. Even for the equal entitlements case, this notion is new, and satisfies APS ≥ MMS, where the inequality is sometimes strict. We also present an alternative definition for the APS as a maximization problem (a fractional version of the MMS), and provide comparisons between the APS and previous notions of fairness. Our main result concerns additive valuations and arbitrary entitlements, for which we provide a polynomial-time algorithm that gives every agent at least a 3/5-fraction of her APS. This algorithm can also be viewed as providing a strategy in a certain natural bidding game, and this strategy secures each agent that uses it at least a 3/5-fraction of her APS, regardless of the strategies used by other agents.