103 research outputs found
Bruhat order, smooth Schubert varieties, and hyperplane arrangements
The aim of this article is to link Schubert varieties in the flag manifold
with hyperplane arrangements. For a permutation, we construct a certain
graphical hyperplane arrangement. We show that the generating function for
regions of this arrangement coincides with the Poincare polynomial of the
corresponding Schubert variety if and only if the Schubert variety is smooth.
We give an explicit combinatorial formula for the Poincare polynomial. Our main
technical tools are chordal graphs and perfect elimination orderings.Comment: 14 pages, 2 figure
From Bruhat intervals to intersection lattices and a conjecture of Postnikov
We prove the conjecture of A. Postnikov that (A) the number of regions in the
inversion hyperplane arrangement associated with a permutation w\in \Sn is at
most the number of elements below in the Bruhat order, and (B) that
equality holds if and only if avoids the patterns 4231, 35142, 42513 and
351624. Furthermore, assertion (A) is extended to all finite reflection groups.
A byproduct of this result and its proof is a set of inequalities relating
Betti numbers of complexified inversion arrangements to Betti numbers of closed
Schubert cells. Another consequence is a simple combinatorial interpretation of
the chromatic polynomial of the inversion graph of a permutation which avoids
the above patterns.Comment: 24 page
Condorcet domains of tiling type
A Condorcet domain (CD) is a collection of linear orders on a set of
candidates satisfying the following property: for any choice of preferences of
voters from this collection, a simple majority rule does not yield cycles. We
propose a method of constructing "large" CDs by use of rhombus tiling diagrams
and explain that this method unifies several constructions of CDs known
earlier. Finally, we show that three conjectures on the maximal sizes of those
CDs are, in fact, equivalent and provide a counterexample to them.Comment: 16 pages. To appear in Discrete Applied Mathematic
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