Unseparated pairs and fixed points in random permutations

Persi Diaconis, Steven N. Evans, Ron Graham

Published 2013-08-25, updated 2014-04-27Version 2

In a uniform random permutation \Pi of [n] := {1,2,...,n}, the set of elements k in [n-1] such that \Pi(k+1) = \Pi(k) + 1 has the same distribution as the set of fixed points of \Pi that 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 \Pi(k+1 mod n) = \Pi(k) + 1 mod n. This latter random set is just the set of fixed points of the commutator [\rho, \Pi], where \rho is the n-cycle (1,2,...,n). We show for a general permutation \eta that, under weak conditions on the number of fixed points and 2-cycles of \eta, the total variation distance between the distribution of the number of fixed points of [\eta,\Pi] and a Poisson distribution with expected value 1 is small when n is large.