Sharp Asymptotics for the Solvability Probability of Random Stable Roommates
For even $n$, let $P_n$ be the probability that independent uniform strict preference lists on $n$ participants admit a stable perfect matching. We prove \[ P_n\sim\frac{e\,2^{1/4}Γ(3/4)}{\sqrtπ}\,n^{-1/4}. \] This establishes Mertens's conjectured exponent of decay, with a leading constant different from his original numerical prediction. The proof starts from Mertens's exact alternating sum over stable permutations. To preserve its cancellation, we construct a common approximation for all cycle structures with the same number of vertices in cycles longer than two. By symmetry, the integrated first-order correction is the same for every such cycle structure, and the remaining errors can be summed in absolute value. The enumeration then reduces the probability to a one-dimensional sum with positive terms.