Higher Specht bases and $q$-series for the cohomology rings of certain Hessenberg varieties

Abstract: It is conjectured (following the Stanley-Stembridge conjecture) that the cohomology rings of regular semisimple Hessenberg varieties yield permutation representations, but the decompositions of the modules are only known in some cases. For the Hessenberg function $h=(h(1),n,\ldots,n)$, the structure of the cohomology ring was determined by Abe, Horiguchi, and Masuda in 2017. We define two new bases for this cohomology ring, one of which is a higher Specht basis, and the other of which is a permutation basis. We also examine the transpose Hessenberg variety, indexed by the Hessenberg function $h' = ((n-1)^{n-m},n^m)$, and show that analogous results hold. Further, we give combinatorial bijections between the monomials in the new basis and sets of $P$-tableaux, motivated by the work of Gasharov, illustrating the connections between the $\mathfrak{S}_n$ action on these cohomology rings and the Schur expansion of chromatic symmetric functions.