Unseparated pairs and fixed points in random permutations

Abstract

In a uniform random permutation Πof [n] := {1,2,...,n}, the set of elements k in [n-1] such that Π(k+1) = Π(k) + 1 has the same distribution as the set of fixed points of Πthat lie in [n-1]. We give three different proofs of this fact using, respectively, an enumeration relying on the inclusion-exclusion principle, the introduction of two different Markov chains to generate uniform random permutations, and the construction of a combinatorial bijection. We also obtain the distribution of the analogous set for circular permutations that consists of those k in [n] such that Π(k+1 mod n) = Π(k) + 1 mod n. This latter random set is just the set of fixed points of the commutator [ρ, Π], where ρis the n-cycle (1,2,...,n). We show for a general permutation ηthat, under weak conditions on the number of fixed points and 2-cycles of η, the total variation distance between the distribution of the number of fixed points of [η,Π] and a Poisson distribution with expected value 1 is small when n is large.

revised to incorporate corrections suggested by referee