The Geometry and Combinatorics of Some Hessenberg Varieties Related to the Permutohedral Variety

Abstract: We construct a concrete isomorphism from the permutohedral variety to the regular semisimple Hessenberg variety associated to the Hessenberg function $h_+(i)=i+1$, $1\le i\le n-1$. In the process of defining the isomorphism, we introduce a sequence of varieties which we call the prepermutohedral varieties. We first determine the toric structure of these varieties and compute the Euler characteristics and the Betti numbers using the theory of toric varieties. Then, we describe the cohomology of these varieties. We also find a natural way to encode the one-dimensional components of the cohomology using the codes defined by Stembridge. Applying the isomorphisms we constructed, we are also able to describe the geometric structure of regular semisimple Hessenberg varieties associated to the Hessenberg function represented by $h_k= (2,3, \cdots, k+1, n,\cdots,n)$, $1\le k\le n-3$. In particular, we are able to write down the cohomology ring of the variety. Finally, we determine the dot representation of the permutation group ${\frak S}_n$ on these Hessenberg varieties.