Rook Placements in Young Diagrams and Permutation Enumeration

Abstract. Given two operators M and N subject to the relation MN −qNM = p, and a word w in M and N , the rewriting of w in normal form is combinatorially described by rook placements in a Young diagram. We give enumerative results about these rook placements, particularly in the case where p = (1−q)/q. This case naturally arises in the context of the PASEP, a random process whose partition function and stationary distribution are expressed using two operators D and E subject to the relation DE− qED = D+E (matrix Ansatz). Using the link obtained by Corteel and Williams between the PASEP, permutation tableaux and permutations, we prove a conjecture of Corteel and Rubey about permutation enumeration. This result gives the generating function for permutations of given size with respect to the number of ascents and occurrences of pattern 13-2, this is also the moments of the q-Laguerre orthogonal polynomials.