Abstract
We study the problem of finding maximum weakly stable matchings when preference lists are incomplete and contain one-sided ties of bounded length. We show that if the tie length is at most L, then it is possible to achieve an approximation ratio of \(1 + (1 - \frac {1}{L})^{L}\). We also show that the same ratio is an upper bound on the integrality gap, which matches the known lower bound. In the case where the tie length is at most 2, our result implies an approximation ratio and integrality gap of \(\frac {5}{4}\), which matches the known UG-hardness result.
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Notes
In the proof of this claim [22, Theorem 19], Huang et al. exhibit a family of instances with 2k men and 2k women such that the corresponding LP has a feasible fractional value of (3/2 − o(1))k. It is asserted that a certain weakly stable matching of size k is a maximum weakly stable matching, but this assertion is incorrect. For the case when k = 2, there exists a weakly stable matching of size 3. Similarly, when k > 2, it can be shown that the maximum size of weakly stable matching is greater than k.
There is also a flaw related to the main result of their paper, which asserts an approximation ratio of \(\frac {5}{4}\) for the special case where ties are one-sided and are restricted to the end of the preference lists. In the derivation of inequalities (11) and (12) in their proof [22, Lemma 16], it is claimed that \(\frac {\delta _{m, w}}{1 + \nu _{w}} \leq \delta _{m, w}\). This claim depends on the unproven assumption that δm, w is non-negative. It is unclear whether this flaw can be fixed. Both flaws have been acknowledged by Huang et al. in a personal communication.
A binary relation ≽ over a set K is said to satisfy antisymmetry if for every k1, k2 ∈ K such that k1 ≽ k2 and k2 ≽ k1, we have k1 = k2.
A binary relation ≽ over a set K is said to satisfy transitivity if for every k1, k2, k3 ∈ K such that k1 ≽ k2 and k2 ≽ k3, we have k1 ≽ k3.
A binary relation ≽ over a set K is said to satisfy totality if for every k1, k2 ∈ K, we have either k1 ≽ k2 or k2 ≽ k1.
The asymmetric part of a binary relation ≽ over a set K is the binary relation ≻ over the set K such that for every k1, k2 ∈ K, we have k1 ≻ k2 if and only if k1 ≽ k2 and \(\lnot (k_{2} \succeq k_{1})\).
The symmetric part of a binary relation ≽ over a set K is the binary relation \(\sim \) over the set K such that for every k1, k2 ∈ K, we have \(k_{1} \sim k_{2}\) if and only if k1 ≽ k2 and k2 ≽ k1.
Some of the literature on stable matching with indifferences does not allow an agent to be indifferent between being matched to an agent and being unmatched. Our formulation of the stable matching problem with one-sided ties and incomplete lists allows for this possibility, since we can have i =j0 for any man i and woman j.
For any positive step size η, the upper bound on pi can be strengthened to pi < 1 + 2η. Later in the paper, we present an algorithm (Algorithm 1) that terminates in a state satisfying (P1) through (P4) with η = 0 (Lemma 13); in that context, we have pi ≤ 1 and not pi < 1.
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We thank the two anonymous referees for their valuable comments, which helped us to significantly improve the presentation.
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This article belongs to the Topical Collection: Special Issue on Algorithmic Game Theory (SAGT 2019)
Guest Editors: Dimitris Fotakis and Vangelis Markakis
This journal article combines the results presented in two conference papers [1, 2]. The first of these papers establishes an approximation ratio of \(1+\protect \frac {1}{e}\). The second establishes the more detailed bound stated in the abstract. The latter bound is increasing in L and approaches \(1+\protect \frac {1}{e}\) as L tends to infinity.
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Lam, CK., Plaxton, C.G. Maximum Stable Matching with One-Sided Ties of Bounded Length. Theory Comput Syst 66, 645–678 (2022). https://doi.org/10.1007/s00224-022-10072-1
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DOI: https://doi.org/10.1007/s00224-022-10072-1