# Random stable roommates

**Conjecture**The probability that a random instance of the stable roommates problem on people admits a solution is .

A *system of preferences* for a graph is a family so that every is a linear ordering of the neighbors of the vertex . We say that *prefers* to if . A perfect matching in is *stable* if there do not exist so that prefers to and prefers to .

A famous theorem of Gale-Shapley [GS] proves that every system of preferences on a complete bipartite graph admits a stable perfect matching. Indeed, they provide an amusing algorithm to construct one. On complete graphs, this problem is known as either the homosexual stable marriage problem, or more commonly, the stable roommate problem. Here there does not always exist a solution (that is, a stable perfect matching), but Irving [I] constructed an algorithm which runs in polynomial time, and outputs a solution if one exists.

Let denote the probability that a random instance of the stable roommates problem has a solution (so the above conjecture asserts that ). The following are the best known asymptotic bounds for (with even) and hold for sufficiently large. The lower bound is due to Pittel [P] and the upper bound to Pittel and Irving [IP]

Mertens [M] did an extensive Monte-Carlo simulation to obtain the above conjecture. Indeed, by guessing at the constant he even offers the stronger conjecture .

## Bibliography

[GS] D. Gale D and L. S. Shapley, College admissions and the stability of marriage, Am. Math. Mon. 69 9-15.

[I] R. W. Irving, An efficient algorithm for the stable roommates problem, J. Algorithms 6 577-95.

[IP] B. Pittel and R. W. Irving, An upper bound for the solvability of a random stable roommates instance, Random Struct. Algorithms 5 465-87.

*[M] S. Mertens, Random stable matchings, J. Stat. Mech. Theory Exp. 2005, no. 10 MathSciNet

[P] B. Pittel, The 'stable roommates' problem with random preferences, Ann. Probab. 21 1441-77

* indicates original appearance(s) of problem.