Combinatorial models for Schubert polynomials

Abstract

Schubert polynomials are a basis for the polynomial ring that represent Schubert classes for the flag manifold. In this paper, we introduce and develop several new combinatorial models for Schubert polynomials that relate them to other known bases including key polynomials and fundamental slide polynomials. We unify these and existing models by giving simple bijections between the combinatorial objects indexing each. In particular, we give a simple bijective proof that the balanced tableaux of Edelman and Greene enumerate reduced expressions and a direct combinatorial proof of Kohnert's algorithm for computing Schubert polynomials. Further, we generalize the insertion algorithm of Edelman and Greene to give a bijection between reduced expressions and pairs of tableaux of the same key diagram shape and use this to give a simple formula, directly in terms of reduced expressions, for the key polynomial expansion of a Schubert polynomial.