Roommates with Convex Preferences

Abstract

Roommate problems with convex preferences always have stable matchings. Efficiency and individual rationality are, moreover, compatible with strategyproofness in such convex roommate problems. Both of these results fail without the assumption of convexity. In the environment under study, preferences are convex if and only if they are single peaked. Any individually rational and convex roommate problem is homomorphic to a marriage market where an agent's gender corresponds to the direction of the agent's top-ranked partner. The existence of stable matchings then follows from the existence of stable matchings in marriage markets. To prove the second existence result, I define an efficient, individually rational, and strategyproof mechanism for convex roommate problems. To calculate outcomes, this mechanism starts with all agents being single and then gradually reassigns agents to better partners by performing minimal Pareto improvements. Whenever it becomes clear that some agent cannot be part of any further Pareto improvement, such an agent is matched.