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Consensus Halving for Sets of Items

Paul W. Goldberg
Alexandros Hollender
Ayumi Igarashi
Warut Suksompong
Conference on Web and Internet Economics (WINE) (2020), pp. 384-397
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Consensus halving refers to the problem of dividing a resource into two parts so that every agent values both parts equally. Prior work has shown that when the resource is represented by an interval, a consensus halving with at most n cuts always exists, but is hard to compute even for agents with simple valuation functions. In this paper, we study consensus halving in a natural setting where the resource consists of a set of items without a linear ordering. When agents have additive utilities, we present a polynomial-time algorithm that computes a consensus halving with at most n cuts, and show that n cuts are almost surely necessary when the agents’ utilities are drawn from probabilistic distributions. On the other hand, we show that for a simple class of monotonic utilities, the problem already becomes PPAD-hard. Furthermore, we compare and contrast consensus halving with the more general problem of consensus k-splitting, where we wish to divide the resource into k parts in possibly unequal ratios, and provide some consequences of our results on the problem of computing small agreeable sets.