Structural Complexities of Matching Mechanisms

We study various novel complexity measures for two-sided matching mechanisms, applied to the two canonical strategyproof matching mechanisms, Deferred Acceptance (DA) and Top Trading Cycles (TTC). Our metrics are designed to capture the complexity of various structural (rather than computational) concerns, in particular ones of recent interest from economics. We consider a canonical, flexible approach to formalizing our questions: define a protocol or data structure performing some task, and bound the number of bits that it requires. Our results apply this approach to four questions of general interest; for matching applicants to institutions, we ask: (1) How can one applicant affect the outcome matching? (2) How can one applicant affect another applicant's set of options? (3) How can the outcome matching be represented / communicated? (4) How can the outcome matching be verified? We prove that DA and TTC are comparable in complexity under questions (1) and (4), giving new tight lower-bound constructions and new verification protocols. Under questions (2) and (3), we prove that TTC is more complex than DA. For question (2), we prove this by giving a new characterization of which institutions are removed from each applicant's set of options when a new applicant is added in DA; this characterization may be of independent interest. For question (3), our result gives lower bounds proving the tightness of existing constructions for TTC. This shows that the relationship between the matching and the priorities is more complex in TTC than in DA, formalizing previous intuitions from the economics literature. Together, our results complement recent work that models the complexity of observing strategyproofness and shows that DA is more complex than TTC. This emphasizes that diverse considerations must factor into gauging the complexity of matching mechanisms.