SPUR is finally over! Instead of continuing my series of blog posts, I thought I’d just link to my paper, Lattice paths and the quadratic coefficient of Kazhdan-Lusztig polynomials, and the first few blog posts should more or less provide enough background to read it.

My project ended up changing direction. The formula I was working with for the quadratic coefficient was so unwieldy that I ended up spending the whole time trying to simplify it, and instead of saying anything about non-negativity I ended up saying something about combinatorial invariance. The combinatorial invariance conjecture, which goes back to Lusztig and, independently, Dyer, says that the Kazhdan-Lusztig polynomial depends only on the poset structure of . In the special case that this was proven in 2006 by Brenti, Caselli, and Marietti. However, the conjecture is still open in general.

In particular, explicit nonrecursive formulas in which each term only depends on poset-theoretic data are not known in general. They are known in the case that the length of the interval is less than or equal to , and there is also such a formula for the coefficient of of where is the identity. The main result of the paper is a formula for the coefficient of of in which all but three of the terms depend only on poset data, which is a simplification of a general formula due to Brenti for in terms of lattice paths. It reduces to

- a formula for the coefficient of of in which all but one of the terms depends only on poset data,
- a formula for the coefficient of of where in which all but one of the terms (but a different term) depends only on poset data (not in the paper), and
- a formula for the coefficient of of where in which every term depends only on poset data.

I believe these formulas are known in some form, but the method of proof is likely to be novel. In any case, the troublesome terms in the above are all essentially coefficients of R-polynomials. If I revisit this project in the future, I will be focusing my attention on these coefficients, and my goal will be to find a poset-theoretic formula for in the length case, the smallest-length case where (to my knowledge) combinatorial invariance is open.