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09-07-2024 | Original Paper

Partitionable choice functions and stability

Authors: Umut Dur, Thayer Morrill, William Phan

Published in: Social Choice and Welfare | Issue 2/2024

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Abstract

We consider the two-sided many-to-one matching problem and introduce a class of preferences reflecting natural forms of complementarities. For example, academic departments hire seniors and then supporting juniors, teams recruit different roles and specialties, starting with the critical ones, and firms hire workers at various levels, starting with the executives. The key feature is that a firm can partition workers into types and prioritize certain types before others. Despite this partitionability requirement of choice functions being weaker than substitutes—an essential condition concerning the existence of a stable assignment—we show that it still guarantees the existence of a stable assignment and is further a maximal domain for such.

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Appendix
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Footnotes
1
This example is reminiscent of Danilov (2003); however, in our environment, hospitals have preferences over arbitrary subsets of all types of agents, whereas he considers matchings that are tuples consisting of only one agent of every type.
 
2
Implicitly, we consider the hospitals’ choice functions collectively instead of individually, and our theoretical exercise is to identify a property of a profile of choice functions that is sufficient to guarantee the existence of a stable assignment. We may view the substitutes domain as a rectangular domain for profiles of choice functions, and the partitionable domain, a non-rectangular (\(\pi \) must be consistent across agents) superset of the substitutes domain.
 
3
A similar exercise is performed with single-peaked preferences: Can the set of alternatives be ordered so that each agent’s preference is single-peaked (with respect to this order)?
 
4
When all agents have the same type, i.e., the partition is trivial, the SDA is equivalent to DA.
 
5
Some variants of substitutability in the matching with contracts literature, such as bilateral substitutability, are equivalent to substitutability when there is only a single contract available for each potential match.
 
6
We need a consistency property, called irrelevance of rejected alternatives, in addition to substitutability to guarantee the existence of stable outcomes (Fleiner 2003; Aygün and Sönmez 2013). Throughout our analysis, we assume the choice functions of each agent on the multi-unit demand side satisfy this consistency property.
 
7
In a transferable utility setting, using insights from tropical geometry, Baldwin and Klemperer (2019) show that competitive equilibria can exist for a far more general class of utility functions than gross substitutes.
 
8
Rostek and Yoder (2020) conduct a similar exercise but with complementary preferences. Their domain and the substitutes domain are unrelated (i.e., have empty intersection), while our partitionability condition includes the latter as a special case.
 
9
Che et al. (2019) considers a large market matching model, and shows that approximately stable matchings exist without the substitutes condition.
 
10
Hatfield et al. (2019) show that in the matching with contracts framework, the cumulative offer process is the unique strategy-proof and stable mechanism when three conditions on multi-unit demand side choice function are satisfied—observable substitutes, observable size monotonicity, and non-manipulability via contractual terms. Moreover, if the choice function of a firm does not satisfy any of these three conditions, then there exist instances in which all other firms have unit demand choice functions such that strategy-proof and stable mechanisms fail to exist. In our setting, matching without contracts, observable substitutes, and observable size monotonicity correspond to substitutes and size monotonicity, respectively. Hence, our discussions on Hatfield and Kojima (2008) also hold here.
 
11
Notice that we do not restrict the number of staff, either doctor or nurse, assigned to a hospital. Instead, each hospital determines its capacity through the choice function.
 
12
That is, it is not blocked by any hospital \(h\in H\) and subset of staff \(\bar{M}\subset M\) such that each staff in \(\bar{M}\) belongs to a group.
 
13
Both Hatfield and Milgrom (2005) and Hatfield and Kojima (2008) consider the matching with contracts framework. The two-sided matching problem is a special case where there is a unique contract term for each medical staff and hospital pair. In our setting, substitutability and irrelevance of rejected alternatives together are sufficient conditions to guarantee the existence of stable matching (Fleiner 2003).
 
14
Substitutability, as well as weaker conditions, i.e., bilateral and unilateral substitutes (Hatfield and Kojima 2010), have been shown to be sufficient conditions in the matching with contracts model. Since, in our setting, each hospital-staff pair have only one possible contract, such conditions are all the same.
 
15
Notice that hospital h’s choice function fails to be substitutable even if it accepts a doctor alone.
 
16
This is reminiscent of the history independence assumption in Kotowski (2019) in the context of a dynamic matching when agents are repeatedly assigned each period. Their property requires that matchings in previous periods do not affect preferences today. Our downstream independence is the opposite—matchings in previous/upstream groups can affect selections in the current group, but later/downstream groups cannot affect previous selections.
 
17
Note that by downstream independence, \(\mu _{g-1}(h)\) is a subset of the chosen staff from \(\mu _{g-1}(h)\cup A_1(h).\)
 
18
Our result in this section still holds if the number of nurses needed to support a doctor differs across doctors.
 
19
For completeness, we provide the proof of Theorem 2 in the Appendix.
 
20
A similar problem occurs if we attempt to reverse the order of SDA with Placeholders and assign nurses first by permuting the roles of the two groups.
 
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Metadata
Title
Partitionable choice functions and stability
Authors
Umut Dur
Thayer Morrill
William Phan
Publication date
09-07-2024
Publisher
Springer Berlin Heidelberg
Published in
Social Choice and Welfare / Issue 2/2024
Print ISSN: 0176-1714
Electronic ISSN: 1432-217X
DOI
https://doi.org/10.1007/s00355-024-01534-1

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