Singularities of schubert varieties within a right cell

Abstract

We describe an algorithm which pattern embeds, in the sense of Woo–Yong, any Bruhat interval of a symmetric group into an interval whose extremes lie in the same right Kazhdan–Lusztig cell. This apparently harmless fact has applications in finding examples of reducible associated varieties of sln-highest weight modules, as well as in the study of W-graphs for symmetric groups, and in comparing various bases of irreducible representations of the symmetric group or its Hecke algebra. For example, we are able to systematically produce many negative answers to a question from the 1980s of Borho–Brylinski and Joseph, which had been settled by Williamson via computer calculations only in 2014.