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Enumerative combinatorics, graph theory, order theory, posets, matroids, designs and other discrete structures. It also includes algebraic, analytic and probabilistic combinatorics.

4 votes
Accepted

Are parabolic Kazhdan-Lusztig polynomials truncations of the usual Kazhdan-Lusztig polynomials?

A quick answer for which standard health warnings apply. For your first question, the answer is positive, as follows (for example) from my paper with Libedinsky here: https://arxiv.org/abs/1702.00459 …
Geordie Williamson's user avatar
8 votes

Combinatorial constructions found by computer

There is a beautiful story involving the faithfulness question of the Burau representation of the braid group. Moody showed that the Burau representation is unfaithful if $n$ (the number of strands) …
14 votes
0 answers
617 views

Chasing a 1950s thesis from the University of Dhaka on block designs

On behalf of a friend I am searching for a thesis on block designs from the 1950s. The details are below. Author: Qazi Motahar Husein (Sometimes Husain or Hussein). Title of the Thesis: Symmetrical …
Geordie Williamson's user avatar
4 votes

Intersection homology for toric varieties

There is a simple and beautiful description in terms of commutative algebra (repeatedly calculating global sections and taking a projective cover). The work of Braden-MacPherson cited by Alexander is …
Geordie Williamson's user avatar
4 votes

Subexpressions of reduced words in Coxeter groups

I have found a much more efficient way of solving this problem on computer. Having asked the question I guess I should provide a brief account. However I feel like the algorithm is technical and not v …
Geordie Williamson's user avatar
9 votes
2 answers
894 views

Subexpressions of reduced words in Coxeter groups

Let $\underline{w} = [s_1, s_2, \dots ,s_n]$ be a reduced expression in a Coxeter group $W$. Given $x$ in $W$ one can consider the set $\Pi(\underline{w},x)$ consisting of all subexpressions of $\unde …
Geordie Williamson's user avatar
25 votes

Is Soergel's proof of Kazhdan-Lusztig positivity for Weyl groups independent of other proofs?

Perhaps I can supplement Jim's answer a little. In the paper "Kazhdan-Lusztig-Polynome und unzerlegbare Bimoduln uber Polynomringen" Soergel shows that there are certain graded indecomposable bimodul …
Geordie Williamson's user avatar