A q-analog of the decomposition of the regular representation of the symmetric group
It is a well-known fact of representation theory that, if the irreducible representations of a finite group are , and is the regular representation formed by acting on itself by left multiplication, then is its decomposition into irreducibles.
I’ve recently discovered a -analog of this fact for that is a simple consequence of some known results in symmetric function theory.
In Enumerative Combinatorics, Stanley defines a generalization of the major index on permutations to standard tableaux. For a permutation of , a descent is a position such that . For instance, has two descents, in positions and . The major index of , denoted , is the sum of the positions of the descents, in this case
To generalize this to standard Young tableaux, notice that is a descent of if and only if the location of occurs after in the inverse permutation . With this as an alternative notion of descent, we define a descent of a tableau to be a number for which occurs in a lower row than . In fact, this is precisely a descent of the inverse of the reading word of , the word formed by reading the rows of from left to right, starting from the bottom row.
As an example, the tableau below has two descents, and , since and occur in lower rows than and respectively:

So . Note that its reading word , and the inverse permutation is , which correspondingly has descents in positions and .
(This is a slightly different approach to the major index than taken by Stanley, who used a reading word that read the columns from bottom to top, starting at the leftmost column. The descents remain the same in either case, since both reading words Schensted insert to give the same standard Young tableau.)
Now, the major index for tableaux gives a remarkable specialization of the Schur functions . As shown in Stanley’s book, we have where the sum is over all standard Young tableaux of shape . When I came across this fact, I was reminded of a similar specialization of the power sum symmetric functions. It is easy to see that an identity that comes up in defining a -analog of the Hall inner product in the theory of Hall-Littlewood symmetric functions. In any case, the power sum symmetric functions are related to the Schur functions via the irreducible characters of the symmetric group , and so we get
This can be simplified to the equation: where denotes the shape of the tableau .
Notice that when we take above, the right hand side is unless is the partition of into all ’s. If is not this partition, setting yields where is the number of standard Young tableaux of shape . Otherwise if , we obtain Recall also that (see e.g. Stanley or Sagan) is equal to the dimension of the irreducible representation of . Thus, these two equations together are equivalent to the fact that, if is the regular representation, which is in turn equivalent to the decomposition of into irreducibles.
Therefore, Equation (1) is a -analog of the decomposition of the regular representation. I’m not sure this is known, and I find it’s a rather pretty consequence of the Schur function specialization at powers of .
EDIT: It is known, as Steven Sam pointed out in the comments below, and it gives a formula for a graded character of a graded version of the regular representation.