Optimal Capacity Modification for Stable Matchings with Ties
Pith reviewed 2026-05-23 17:42 UTC · model grok-4.3
The pith
The minimum total increase in hospital capacities to ensure a strongly stable matching can be found in polynomial time.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
In the Hospitals/Residents problem with ties in hospital preferences, the MINSUM problem of minimizing the total capacity increase across hospitals to guarantee a strongly stable matching admits a polynomial-time algorithm. An analog of the rural hospitals theorem also holds in this MINSUM augmentation setting. The cost-weighted version of MINSUM is NP-hard even for zero-one costs, while the version with forced edges remains polynomial-time solvable. In contrast, MINMAX is NP-hard in general, but when ties have length at most ℓ+1 a polynomial-time method exists that increases every quota by at most ℓ and returns the residents-optimal strongly stable matching among all such augmentations.
What carries the argument
The MINSUM capacity-augmentation problem for strong stability in the Hospitals/Residents model with ties.
If this is right
- MINSUM admits a polynomial-time algorithm.
- An analog of the rural hospitals theorem holds for MINSUM augmentations.
- The cost version of MINSUM is NP-hard and inapproximable even with zero-one costs.
- With forced edges the MINSUM problem remains polynomial-time solvable.
- When ties have length at most ℓ+1, a polynomial-time method increases each quota by at most ℓ and returns the residents-optimal strongly stable matching.
Where Pith is reading between the lines
- The rural hospitals analog may extend to other quota-modification settings that preserve strong stability.
- The bounded-tie algorithm supplies an explicit bound on the per-hospital increase needed when preference lists are short.
- The NP-hardness of MINMAX suggests that total-sum minimization is the more tractable route for capacity planning under strong stability.
Load-bearing premise
That raising only hospital quotas, without altering preferences or resident constraints, is always enough to produce a strongly stable matching in the modified instance.
What would settle it
An explicit Hospitals/Residents instance with ties where no finite total quota increase produces a strongly stable matching, or where the claimed polynomial-time procedure fails to return the minimum total increase.
Figures
read the original abstract
We consider the Hospitals/Residents (HR) problem in the presence of ties in hospital preferences. Among the three notions of stability, namely weak stability, strong stability, and super-stability, we focus on the notion of strong stability. Strong stability has many desirable properties, both theoretically and in practice; however, its existence is not guaranteed. In this paper, our objective is to optimally increase the quotas of hospitals to ensure that a strongly stable matching exists in the modified instance. We explore two natural optimization criteria: (i) minimizing the total capacity increase across all hospitals (MINSUM) and (ii) minimizing the maximum capacity increase for any hospital (MINMAX). We show that the MINSUM problem admits a polynomial-time algorithm. We also establish an analog of the well-known rural hospitals theorem [Gale & Sotomayor, 1985; Roth, 1986], adapted to the MINSUM augmentation setting. We consider a generalization of the MINSUM problem in which each hospital incurs a cost per unit increase in its quota. We show that the cost version of the MINSUM problem is NP-hard and inapproximable within any multiplicative factor, even if the costs are zero or one. For the MINSUM objective with a set of forced edges, we give a polynomial-time algorithm. In contrast to the above results for the MINSUM problem, we show that the MINMAX problem is NP-hard. When hospital preference lists have ties of length at most $\ell+1$, we give a polynomial-time algorithm that increases each hospital's quota by at most $\ell$, ensuring the resulting instance admits a strongly stable matching. Moreover, among all such augmentations, our algorithm outputs the best strongly stable matching from the residents' perspective.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript studies the Hospitals/Residents problem with ties only in hospital preference lists, focusing on strong stability. It introduces two quota-augmentation problems to guarantee existence of a strongly stable matching: MINSUM (minimize total capacity increase) and MINMAX (minimize the maximum increase at any hospital). The central claims are a polynomial-time algorithm for unweighted MINSUM, an analog of the rural hospitals theorem in this augmentation setting, NP-hardness (and inapproximability) for the weighted MINSUM variant and for MINMAX, a polynomial-time algorithm for MINSUM with forced edges, and—for preference lists whose ties have length at most ℓ+1—a polynomial-time algorithm that augments each quota by at most ℓ while returning the resident-optimal strongly stable matching in the augmented instance.
Significance. If the algorithmic claims hold, the work is significant: it supplies the first efficient method for minimally enlarging capacities to restore strong stability, cleanly separates the tractable MINSUM case from the intractable MINMAX and weighted cases, and extends the classical rural hospitals theorem to the augmentation setting. The bounded-tie-length result for MINMAX further delineates the complexity landscape and yields a resident-optimal solution, which is a desirable property in applications.
minor comments (3)
- [Abstract] The abstract states that the MINSUM algorithm is polynomial-time but does not indicate its degree; adding a concrete bound (e.g., O(n^3)) would help readers assess practicality.
- The statement of the rural-hospitals analog should explicitly list the quantities that remain invariant (e.g., the set of matched residents or the number of residents assigned to each hospital) so that the precise analogy is immediately clear.
- In the bounded-tie-length result, the paper should clarify whether the ℓ-increase bound is tight (i.e., whether there exist instances requiring an increase of exactly ℓ at some hospital).
Simulated Author's Rebuttal
We thank the referee for the positive summary, recognition of the significance of the results, and recommendation of minor revision. No major comments appear in the report, so we have no points to address point-by-point.
Circularity Check
No significant circularity detected
full rationale
The paper presents new polynomial-time algorithms for MINSUM quota augmentation to guarantee strong stability, an adapted rural hospitals theorem, and contrasting NP-hardness results for MINMAX and the cost version. These are constructive algorithmic results and complexity classifications. No derivation reduces by construction to fitted parameters, self-definitional loops, or load-bearing self-citations; the central claims rest on independent algorithmic constructions rather than renaming or smuggling prior results.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Standard definition of strong stability in the Hospitals/Residents model with ties in hospital preferences
- domain assumption Existence of a strongly stable matching after suitable quota increases is always achievable
Reference graph
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