We consider fair allocation of indivisible items under an additional constraint: there is an undirected graph describing the relationship between the items, and each agent's share must form a connected subgraph of this graph. This framework captures, e.g., fair allocation of land plots, where the graph describes the accessibility relation among the plots. We focus on agents that have additive utilities for the items, and consider several common fair division solution concepts, such as proportionality, envy-freeness and maximin share guarantee. While finding good allocations according to these solution concepts is computationally hard in general, we design efficient algorithms for special cases where the underlying graph has simple structure, and/or the number of agents-or, less restrictively, the number of agent types-is small. In particular, despite non-existence results in the general case, we prove that for acyclic graphs a maximin share allocation always exists and can be found efficiently.

Abstract. We study Pareto optimal matchings in the context of house allocation problems. We present an O( √ nm) algorithm, based on Gale's Top Trading Cycles Method, for finding a maximum cardinality Pareto optimal matching, where n is the number of agents and m is the total length of the preference lists. By contrast, we show that the problem of finding a minimum cardinality Pareto optimal matching is NP-hard, though approximable within a factor of 2. We then show that there exist Pareto optimal matchings of all sizes between a minimum and maximum cardinality Pareto optimal matching. Finally, we introduce the concept of a signature, which allows us to give a characterization, checkable in linear time, of instances that admit a unique Pareto optimal matching.

Abstract. We study Pareto optimal matchings in the context of house allocation problems. We present an O( √ nm) algorithm, based on Gale's Top Trading Cycles Method, for finding a maximum cardinality Pareto optimal matching, where n is the number of agents and m is the total length of the preference lists. By contrast, we show that the problem of finding a minimum cardinality Pareto optimal matching is NP-hard, though approximable within a factor of 2. We then show that there exist Pareto optimal matchings of all sizes between a minimum and maximum cardinality Pareto optimal matching. Finally, we introduce the concept of a signature, which allows us to give a characterization, checkable in linear time, of instances that admit a unique Pareto optimal matching.

Abstraet. In the eontext of eoalition formation games a player eyaluates a partition on the basis of the set she belongs to, For this eyaluation to be possible, players are supposed to haye preferenees Oyer sets to whieh they eould belong. In this paper, we suggest two extensions of .preferenees oyer indiyiduals to preferenees oyer sets, For the first one, deriyed from the most preferred member of a set, it is shown that a striet eore partition always exists if the original preferenees are striet and a simple algorithm for the eomputation of one striet eore partition is deriyed. This algorithm tums out to be strategy proof. Th'e seeond extension, based on the least preferred member of a set, produces solutions yery similar to those for the stable roommates problem.

No abstract