Symmetric distribution of maj over des in pattern avoidance classes

Question

It appears from computation to be the case (and would prove at least one clause of a conjecture advanced by Bruce Sagan and collaborators in a recent preprint) that in some pattern avoidance classes of permutations, the distribution of the major index is symmetric among permutations with a given descent number. For instance, $$\sum_{S_6(1234)} q^{maj(\sigma)}t^{des(\sigma)} = \dots + (10 q^6 + 35 q^7 + 66 q^8 + 80 q^9 + 66 q^{10} + 35 q^{11} + 10 q^{12}) t^3 + \dots.$$

As you can see, the coefficient of $t^3$ is a symmetric polynomial.

This is not the case for all avoidance classes: for instance, $$\sum_{S_6(2134)} q^{maj(\sigma)}t^{des(\sigma)} = \dots + (4 q^6 + 21 q^7 + 42 q^8 + 61 q^9 + 56 q^{10} + 35 q^{11} + 10 q^{12}) t^3 + \dots.$$

Before I start hammering at this, I was wondering if this was known to be the case for any particular avoidance classes, and if so, which. Since I have not even found any papers that seem to deal with the subject, pointers to one you know of would also be gratefully received.

Answer 1

Let $w=a_1 a_2\cdots a_n\in S_n$. Set $w' =n+1-a_n,n+1-a_{n-1},\dots,n+1-a_1$. Then (1) $\mathrm{des}(w)=\mathrm{des}(w')$, (2) $\mathrm{maj}(w)+\mathrm{maj}(w')=dn$, where $d=\mathrm{des}(w)$, and (3) $\mathrm{is}(w)=\mathrm{is}(w')$, where $\mathrm{is}(w)$ denotes the length of the longest increasing subsequence of $w$. This implies the suggested symmetry property for $S_n(12\cdots p)$. Since the map $w\mapsto w'$ also preserves the length of the longest decreasing subsequence (in fact, it preserves the RSK insertion tableau), the symmetry property also holds for $S_n(12\cdots p, q\cdots 21)$.