On an open problem of Green and Losonczy: exact enumeration of freely braided permutations

Abstract

Recently, Green and Losonczy~ GL1,GL2 introduced freely braided permutation as a special class of restricted permutations has arisen in representation theory. The freely braided permutations were introduced and studied as the upper bound for the number of commutation classes of reduced expressions for an element of a simply laced Coxeter group is achieved if and only if when the element is freely braided. In this paper, we prove that the generating function for the number of freely braided permutations in S n is given by (1-3x-2x 2 +(1+x)√ 1-4x) / (1-4x-x 2 +(1-x 2)√ 1-4x).