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arXiv · 2411.10284

Optimal Capacity Modification for Stable Matchings with Ties

Keshav Ranjan·Meghana Nasre·Prajakta Nimbhorkar
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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.

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Keshav Ranjan, Meghana Nasre, Prajakta Nimbhorkar. 2026-04-14. Optimal Capacity Modification for Stable Matchings with Ties. https://arxiv.org/abs/2411.10284

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