Séminaire Lotharingien de Combinatoire, 82B.41 (2019), 7 pp.

Ezgi Kantarcı Oğuz

Descent polynomials, peak polynomials and an involution on permutations

Abstract. The size of the set of all permutations of n with a given descent set is a polynomial in n, called the descent polynomial. Similarly, the size of the set of all permutations of n with a given peak set, adjusted by a power of 2 gives a polynomial in n, called the peak polynomial. We give a unitary expansion of descent polynomials in terms of peak polynomials. Then we use this expansion, along with an involution that flips the initial segment of a permutation, to give a combinatorial interpretation of the coefficients of the peak polynomial in a binomial basis, thus giving a new proof of the peak polynomial positivity conjecture.


Received: November 15, 2018. Accepted: February 17, 2019. Final version: April 1, 2019.

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