Torus fixed point sets of Hessenberg Schubert varieties in regular semisimple Hessenberg varieties

Abstract

It is well-known that the $T$-fixed points of a Schubert variety in the flag variety $GL_n({\mathbb C})/B$ can be characterized purely combinatorially in terms of Bruhat order on the symmetric group ${\mathfrak{S}}_n$. In a recent preprint, Cho, Hong, and Lee give a combinatorial description of the $T$-fixed points of Hessenberg analogues of Schubert varieties (which we call Hessenberg Schubert varieties) in a regular semisimple Hessenberg variety. This note gives an interpretation of their result in terms of Bruhat order by making use of a partition of the symmetric group defined using so-called subsets of Weyl type. The Appendix, written by Michael Zeng, proves a lemma concerning subsets of Weyl type which is required in our arguments.