# What makes a D_0 graph Schur positive?

Research paper by **Jonah Blasiak**

Indexed on: **13 Nov '14**Published on: **13 Nov '14**Published in: **Mathematics - Combinatorics**

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#### Abstract

We define a D_0 graph to be a graph whose vertex set is a subset of
permutations of n, with edges of the form ...bac... <--> ...bca... or ...acb...
<--> ...cab... (Knuth transformations), or ...bac... <--> ...acb... or
...bca... <--> ...cab... (rotation transformations), such that whenever the
Knuth and rotation transformations at positions i-1, i, i+1 are available at a
vertex, exactly one of these is an edge. The generating function of such a
graph is the sum of the quasisymmetric functions associated to the descent sets
of its vertices. Assaf studied D_0 graphs in the paper *Dual equivalence and
Schur positivity* and showed that they provide a rich source of examples of the
D graphs defined in the paper *Dual equivalence graphs and a combinatorial
proof of LLT and Macdonald positivity*. A key construction of Assaf expresses
the coefficient of q^t in an LLT polynomial as the generating function of a
certain D_0 graph. LLT polynomials are known to be Schur positive by work of
Grojnowski-Haiman, and experimentation shows that many D_0 graphs have Schur
positive generating functions, which suggests a vast generalization of LLT
positivity in this setting.
As part of a series of papers, we study D_0 graphs using the Fomin-Greene
theory of noncommutative Schur functions. We construct a D_0 graph whose
generating function is not Schur positive by solving a linear program related
to a certain noncommutative Schur function. We go on to construct a D graph on
the same vertex set as this D_0 graph.