Web permutations, Seidel triangle and normalized $\gamma$-coefficients

TL;DR

This paper proves a conjecture relating web permutations to the Seidel triangle and normalized gamma-coefficients, and provides combinatorial interpretations for these coefficients.

Contribution

It confirms the enumerative conjecture of Hwang-Jang-Oh and resolves open problems by Xu and Zeng regarding combinatorial interpretations.

Findings

Proved the web permutation enumeration conjecture.

Provided a combinatorial interpretation of normalized gamma-coefficients.

Connected web permutations with cycle-up-down permutations.

Abstract

The web permutations were introduced by Hwang, Jang and Oh to interpret the entries of the transition matrix between the Specht and $\mathrm{SL}_2$-web bases of the irreducible $\S_{2n}$-representation indexed by $(n,n)$. They conjectured that certain classes of web permutations are enumerated by the Seidel triangle. Using generating functions, Xu and Zeng showed that enumerating web permutations by the number of drops, fixed points and cycles gives rise to the normalized $\gamma$-coefficients of the $(\alpha,t)$-Eulerian polynomials. They posed the problems to prove their result combinatorially and to find an interpretation of the normalized $\gamma$-coefficients in terms of cycle-up-down permutations. In this work, we prove the enumerative conjecture of Hwang-Jang-Oh and answer the two open problems proposed by Xu and Zeng.