This framework will be illustrated by a simple
combinatorial proof and interpretation of Taylor's formula in Section 2.1 and equations (5) and (6) in Sections 2.2 and 3.
A
combinatorial proof of Theorem 1.2 has been also obtained in [17] for the non-factorial case, as well as an analogous determinant formula for skew flagged Grothendieck polynomials, special cases of which arise as the Grothendieck polynomials associated to 321-avoiding permutations [1] and vexillary permutations.
It would be nice to have a direct
combinatorial proof of Corollary 4.
Quinn: Proofs that Really Count, The Art of
Combinatorial Proof, Mathematical Association of America, Washington, D.C., 2003.
The only known
combinatorial proof of of the unimodality of q-binomial coefficients is given by O'Hara in [O'H] (see also [SZ, Zei]).
Bressoud [4] gave a
combinatorial proof of Schur's 1926 theorem by establishing a one-to-one correspondence between the two types of partitions counted in the theorem.
In math, such a tangible breakdown is called a
combinatorial proof. Ramanujan's work, and Ono's after it, relied on more-abstract proofs of divisibility.
Zeilberger, A
combinatorial proof of Bass's evaluations of the Ihara-Selberg zeta function for graphs, Trans.
Thanks to it, we obtain a
combinatorial proof of what was left as an open question in [2]: the symmetric distribution of the initial rise and lower contacts of intervals.
We can give a similar
combinatorial proof of Theorem 4.1 in the special case when t = 2.
Such a map would provide a
combinatorial proof of the major index side of (2).
In Section 6 we propose some open problems which will lead to a
combinatorial proof of the Selberg integral formula.