On Permutation Weights and $q$-Eulerian Polynomials

Weights of permutations were originally introduced by Dugan, Glennon, Gunnells, and Steingr\'imsson [2] in their study of the combinatorics of tiered trees. Given a permutation $\sigma$ viewed as a sequence of integers, computing the weight of $\sigma$ involves recursively counting descents of certain subpermutations of $\sigma$. Using this weight function, one can define a $q$-analog of the Eulerian polynomials, $E_n(x,q)$. We prove two main results regarding weights of permutations and the polynomials $E_n(x,q)$. First, we show that the coefficients of $E_n(x, q)$ stabilize as $n$ goes to infinity, which was conjectured by [2], and enables the definition of the formal power series $W_d(t)$ by [2] with interesting combinatorial properties. Second, we derive a recurrence relation for $E_n(x, q)$, similar to the known recurrence for the classical Eulerian polynomials $A_n(x)$.