Unsolvability and Beyond in Many-To-Many Non-Bipartite Stable Matching

We study the Stable Fixtures problem, a many-to-many generalisation of the classical non-bipartite Stable Roommates matching problem. Building on the foundational work of Tan on stable partitions, we extend his results to this significantly more general setting and develop a rich framework for understanding stable structures in many-to-many contexts. Our main contribution, the notion of a generalised stable partition (GSP), not only characterises the solution space of this problem, but also serves as a versatile tool for reasoning about ordinal preference systems with capacity constraints. We show that GSPs can be computed efficiently and provide an elegant representation of a problem instance, tightly characterising its preference structure and succinctly certifying the existence and non-existence of stable matchings. Leveraging a connection to stable half matchings, we also establish a non-bipartite analogue of the Rural Hospitals Theorem for stable half-matchings and GSPs, and connect our results to recent work on near-feasible matchings, providing a simpler algorithm and tighter analysis for this problem. Our work also addresses the computational challenges of finding optimal stable half-matchings and GSPs, presenting a flexible integer linear programming model for various objectives. Beyond theoretical insights, we conduct the first empirical analysis of random Stable Fixtures instances, uncovering surprising results, such as the impact of capacity functions on the solvability likelihood. Our work not only unifies and extends classical and recent perspectives on stability in non-bipartite stable matching but also establishes new tools, techniques, and directions for advancing the study of stable matchings and their applications.