TTC Domains

We study the classical object reallocation problem under strict preferences, with a focus on characterizing "TTC domains" -- preference domains on which the Top Trading Cycles (TTC) mechanism is the unique mechanism satisfying individual rationality, Pareto efficiency, and strategyproofness. We introduce a sufficient condition for a domain to be a TTC domain, which we call the top-two condition. This condition requires that, within any subset of objects, if two objects can each be most-preferred, they can also be the top-two most-preferred objects (in both possible orders). A weaker version of this condition, applying only to subsets of size three, is shown to be necessary. These results provide a complete characterization of TTC domains for the case of three objects, unify prior studies on specific domains such as single-peaked and single-dipped preferences, and classify several previously unexplored domains as TTC domains or not.