The stable marriage theorem of Gale and Shapley states that for n men and n women there always exists a stable marriage scheme, that is, a set of marriages such that no man and woman mutually prefer one another to their partners. The stable marriage theorem was generalized in two directions: the stable roommates problem is the ‘one-sided’ version, where any two agents on the market can form a partnership. The generalization by Kelso and Crawford is in the ‘two-sided’ model, but on one side of the market agents have a so-called substitutable choice function, and stability is interpreted in a natural way. It turned out that even if both sides of the market have substitutable choice functions, there still exists a stable assignment. The latter version contains the ‘many-to-many’ model where up to a personal quota, polygamy is allowed for both men and women in the two-sided market. The goal of this work is to solve the stable partnership problem, a generalization of the one-sided model with substitutable choice functions. We do not quite reach that: besides substitutability, we also need for the result the so called increasing property (or, in other terminology, the law of aggregate demand). Luckily, choice functions in well-known stable matching theorems comply with this property. The main result is a generalization of Irving's algorithm, the first efficient method to solve the stable roommates problem. We deduce from the algorithm a generalization of Tan's result on characterizing the existence of a stable matching and prove a generalization of the rural hospital theorem and the so-called splitting property of many-to-many stable matchings. We show that our algorithm is linear-time in some sense and indicate that for general (i.e. not necessarily increasing) substitutable choice functions the stable partnership problem is NP-complete.
