Subword complexes and nil-Hecke moves

Abstract

For a finite Coxeter group W, a subword complex is a simplicial complex associated with a pair (Q, \rho), where Q is a word in the alphabet of simple reflections, \rho is a group element. We describe the transformations of such a complex induced by nil-moves and inverse operations on Q in the nil-Hecke monoid corresponding to W. If the complex is polytopal, we also describe such transformations for the dual polytope. For W simply-laced, these descriptions and results of \cite{Go} provide an algorithm for the construction of the subword complex corresponding to (Q, \rho) from the one corresponding to (\delta(Q), \rho), for any sequence of elementary moves reducing the word Q to its Demazure product \delta(Q). The former complex is spherical if and only if the latter one is the (-1)-sphere.