Alexander Postnikov

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Recently, Berenstein et al. have proposed a duality between a sector of N = 4 super-Yang-Mills theory with large R-charge J , and string theory in a pp-wave background. In the limit considered, the effective ’t Hooft coupling has been argued to be λ = g YMN/J 2 = 1/(μpα). We study Yang-Mills theory at small λ (large μ) with a view to reproducing string(More)
The aim of this paper is to discuss a relationship between total positivity and planar directed networks. We show that the inverse boundary problem for these networks is naturally linked with the study of the totally nonnegative Grassmannian. We investigate its cell decomposition, where the cells are the totally nonnegative parts of the matroid strata. The(More)
The volume and the number of lattice points of the permutohedron Pn are given by certain multivariate polynomials that have remarkable combinatorial properties. We give 3 different formulas for these polynomials. We also study a more general class of polytopes that includes the permutohedron, the associahedron, the cyclohedron, the Stanley-Pitman polytope,(More)
We give an algebro-combinatorial proof of a general version of Pieri’s formula following the approach developed by Fomin and Kirillov in the paper “Quadratic algebras, Dunkl elements, and Schubert calculus.” We prove several conjectures posed in their paper. As a consequence, a new proof of classical Pieri’s formula for cohomology of complex flag manifolds,(More)
We investigate several hyperplane arrangements that can be viewed as deformations of Coxeter arrangements. In particular, we prove a conjecture of Linial and Stanley that the number of regions of the arrangement xi − xj = 1, 1 ≤ i < j ≤ n, is equal to the number of alternating trees on n + 1 vertices. Remarkably, these numbers have several additional(More)
This paper presents a formula for products of Schubert classes in the quantum cohomology ring of the Grassmannian. We introduce a generalization of Schur symmetric polynomials for shapes that are naturally embedded in a torus. Then we show that the coefficients in the expansion of these toric Schur polynomials, in terms of the regular Schur polynomials, are(More)
The aim of this paper is to initiate the study of alcoved polytopes, which are polytopes arising from affine Coxeter arrangements. This class of convex polytopes includes many classical polytopes, for example, the hypersimplices. We compare two constructions of triangulations of hypersimplices due to Stanley and Sturmfels and explain them in terms of(More)
Definition. An intransitive tree or alternating tree T on the set of vertices [n] :=[1, 2, ..., n] is a tree satisfying the following condition: if 1 i< j<k n then [i, j ] and [ j, k] cannot both be edges in T. In other words, for every path i1 , i2 , i3 , i4 , ... in T we have i1<i2>i3<i4> } } } or i1>i2<i3>i4< } } } . These trees first appear in the work(More)