The bigraded Frobenius series of the wreath product superspace coinvariant rings

Abstract

We determine the bigraded Frobenius series of the superspace coinvariant ring $R^{(1,1)}_{\mathbb{Z}_r\wr\mathfrak{S}_n}$ for the wreath products $\mathbb{Z}_r\wr\mathfrak{S}_n$, which is the quotient of the ring $\mathbb{C}[x_1,\ldots,x_n]\otimes\bigwedge[θ_1,\ldots,θ_n]$ of polynomial-valued differential forms by the ideal generated by diagonal $\mathbb{Z}_r\wr\mathfrak{S}_n$-invariants with vanishing constant term. The bigraded multiplicity of each irreducible character is a sum over standard Young $r$-tableaux of an expression determined by the flag major index and flag descent number. This refines the Frobenius series graded by fermionic degree found by Bhattacharya--Rhoades (2026). It specializes at $r=2$ to the hyperoctahedral group $\mathfrak{B}_n$ case, and it specializes at $r=1$ to recover the theorem of Murai--Rhoades--Wilson (2025) on the symmetric group $\mathfrak{S}_n$. We give new formulas for the bigraded multiplicities of hook characters of the superspace coinvariant ring in types $A$ and $B$, yielding upper bounds on trigraded multiplicities of the sign character of $R^{(1,2)}_{\mathfrak{S}_n}$ and the determinant character of $R^{(1,2)}_{\mathfrak{B}_n}$. As a corollary, we deduce a conjecture of F. Bergeron (2024) on hook components of $R^{(1,1)}_{\mathfrak{S}_n}$.