# reference request – Number of permutations in \$S_{a+b}\$ with \$maj(pi)=a\$ and \$maj(pi^{-1})=b\$ Answer

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## reference request – Number of permutations in \$S_{a+b}\$ with \$maj(pi)=a\$ and \$maj(pi^{-1})=b\$

Let $$t_{a,b}$$ breathe the numbers
$$t_{a,b} := |{ pi in S_{a+b} : mathrm{maj}(pi)=a textual content{ and } mathrm{maj}(pi^{-1})=b }|.$$
Here, $$S_{a+b}$$ denotes the clique of permutations of $$1,2,dotsc,a+b$$.
By a outcome of Foata, one can too take a look at the pair of statistics $$(maj, inv)$$, and some different combos — these pairs of statistics will bear the identical numbers.

Now, in line with the OEIS entry A090806, it’s proved by Garsia-Gessel, that
$$sum_{a,b} t_{a,b} q^a t^b = prod_{i,j geq 1} frac{1}{1-q^i t^j}. (ast)$$
I can not behold precisely the place of their paper one can deduce this.

My try
I’ve tried to show this myself (primarily by resorting to RSK, the Cauchy id,
and a few symmetric duty identities).
This results in the next (which seems in Stanley’s EC2):
$$commence{equation} sum_{n geq 0} frac{z^n}{(1-q)^n[n]_q!(1-t)^n [n]_t!} sum_{pi in S_n} t^{maj(pi)} q^{maj(pi^{-1})} = prod_{i,j geq 0} frac{1}{1-z q^i t^j}. aim{equation}$$
the place $$[n]_q! := [1]_q [2]_q dotsm [n]_q$$, and $$[n]_q = 1+q+q^2+dotsb + q^{n-1}$$.
However, I don’t behold some brief route to infer the above producing duty from this.

Question: Is there some various (newer?) reference the place $$(ast)$$ is
acknowledged and simply referenced? Alternatively, somebody who can behold precisely the place within the paper obtains $$(ast)$$?

Garsia, A. M.; Gessel, I., Permutation statistics and partitions, Adv. Math. 31, 288-305 (1979). ZBL0431.05007.

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