Combinatorics of the permutation tableaux of type B

Abstract

Permutation tableaux are combinatorial objects related with permutations and various statistics on them. They appeared in connection with total positivity in Grassmannians, and stationary probabilities in a PASEP model. In particular they gave rise to an interesting q-analog of Eulerian numbers. The purpose of this article is to study some combinatorial properties of type B permutation tableaux, defined by Lam and Williams, and links with signed permutation statistics. We show that many of the tools used for permutation tableaux generalize in this case, including: the Matrix Ansatz (a method originally related with the PASEP), bijections with labeled paths and links with continued fractions, bijections with signed permutations. In particular we obtain a q-analog of the type B Eulerian numbers, having a lot in common with the previously known q-Eulerian numbers: for example they have a nice symmetry property, they have the type B Narayana numbers as constant terms. The signed permutation statistics arising here are of several kinds. Firstly, there are several variants of descents and excedances, and more precisely of flag descents and flag excedances. Other statistics are the crossings and alignments, which generalize a previous definition on (unsigned) permutations. There are also some pattern-like statistics arising from variants of the bijection of Fran\ccon and Viennot.