## 1 Introduction

Let *n* be a positive integer. *Superspace* of rank *n* (over the ground field ${\mathbb {Q}}$) is the tensor product

of a rank *n* polynomial ring with a rank *n* exterior algebra. The ring $\Omega _n$ carries a ‘diagonal’ action of the symmetric group ${\mathfrak {S}}_n$ on *n* letters, viz.

which turns $\Omega _n$ into a bigraded ${\mathfrak {S}}_n$-module by considering *x*-degree and $\theta $-degree separately.

The ring $\Omega _n$ appears in physics, where the ‘bosonic’ $x_i$ variables model the states of bosons and the ‘fermionic’ $\theta _i$ variables model the states of fermions [Reference Peskin and Schroeder22]. A number of recent papers in algebraic combinatorics consider ${\mathfrak {S}}_n$-modules constructed with a mix of commuting and anticommuting variables [Reference Bergeron2, Reference Billey, Rhoades and Tewari5, Reference Kroes and Rhoades17, Reference Rhoades and Wilson27, Reference Zabrocki34]. The Fields Institute Combinatorics Group made the tantalising conjecture (see [Reference Zabrocki34]) that, if $\langle (\Omega _n)^{{\mathfrak {S}}_n}_+ \rangle \subseteq \Omega _n$ denotes the ideal generated by ${\mathfrak {S}}_n$-invariants with vanishing constant term, we have an ${\mathfrak {S}}_n$-module isomorphism

where ${\mathcal {OP}}_n$ denotes the family of all ordered set partitions of $[n] = \{1, \dots , n \}$ (with its natural permutation action of ${\mathfrak {S}}_n$) and ${\mathrm {sign}}$ is the 1-dimensional sign representation of ${\mathfrak {S}}_n$. Despite significant progress [Reference Swanson and Wallach31], the conjecture (1.3) has remained out of reach; even proving $\Omega _n/ \langle (\Omega _n)^{{\mathfrak {S}}_n}_+ \rangle $ has the expected vector space dimension remains open.

The *Vandermonde determinant* $\delta _n \in {\mathbb {Q}}[x_1, \dots , x_n]$ is the polynomial

where $\varepsilon _n = \sum _{w \in {\mathfrak {S}}_n} {\mathrm {sign}}(w) \cdot w \in {\mathbb {Q}}[{\mathfrak {S}}_n]$ is the antisymmetrising element of the symmetric group algebra. Given positive integers $k \leq n$, the authors defined [Reference Rhoades and Wilson27] the following extension $\delta _{n,k}$ of the Vandermonde to superspace:

When $k = n$, we recover the classical Vandermonde: $\delta _{n,n} = \delta _n$. The $\delta _{n,k}$ may be used to build bigraded ${\mathfrak {S}}_n$-stable quotient rings ${\mathbb {W}}_{n,k}$ of $\Omega _n$ as follows.

For $1 \leq i \leq n$, the partial derivative operator $\partial /\partial x_i$ acts naturally on the polynomial ring ${\mathbb {Q}}[x_1, \dots , x_n]$ and, by treating the $\theta _1, \ldots , \theta _n$ as constants, on the ring $\Omega _n$. We also have a ${\mathbb {Q}}[x_1,\dots ,x_n]$-linear operator $\partial /\partial \theta _i$ on $\Omega _n$ defined by

where $1 \leq j_1, \dots , j_r \leq n$ are any distinct indices.Footnote ^{1} These operators satisfy the defining relations of $\Omega _n$, namely

for all $1 \leq i, j \leq n$. Given $f \in \Omega _n$, we, therefore, have a well-defined operator $\partial f$ on $\Omega _n$ given by replacing each $x_i$ in *f* with $\partial /\partial x_i$ and each $\theta _i$ in *f* with $\partial /\partial \theta _i$. This gives rise to an action, denoted $\odot $, of $\Omega _n$ on itself:

### Definition 1.1 (Rhoades-Wilson [Reference Rhoades and Wilson27]).

Given positive integers $k \leq n$, let ${\mathrm {ann}} \, \delta _{n,k} \subseteq \Omega _n$ be the annihilator of the superspace Vandermonde $\delta _{n,k}$ under the $\odot $-action:

We let ${\mathbb {W}}_{n,k}$ be the quotient of $\Omega _n$ by this annihilator:

When $k = n$, the ring ${\mathbb {W}}_{n,n}$ may be identified with the singly graded type A coinvariant ring

where $e_d = e_d(x_1, \dots , x_n)$ is the degree *d* elementary symmetric polynomial. Borel [Reference Borel4] proved that $R_n$ presents the cohomology $H^{\bullet }(\mathcal {F \ell }_n; {\mathbb {Q}})$ of the variety $\mathcal {F \ell }_n$ of complete flags in ${\mathbb {C}}^n$. Since $\mathcal {F \ell }_n$ is a smooth compact complex manifold, this means that the ring $R_n$ satisfies Poincaré Duality and the Hard Lefschetz theorem. In this paper, we provide a simple generating set (Definition 4.6, Theorem 4.12) of the ideal ${\mathrm {ann}} \, \delta _{n,k}$.

The quotient ring ${\mathbb {W}}_{n,k}$ is a bigraded ${\mathfrak {S}}_n$-module; we let $({\mathbb {W}}_{n,k})_{i,j}$ denote its bihomogeneous piece in *x*-degree *i* and $\theta $-degree *j*. In [Reference Rhoades and Wilson27], the following facts were proven about this module. Let ${\mathrm {grFrob}}({\mathbb {W}}_{n,k}; q, z)$ be the bigraded Frobenius image of ${\mathbb {W}}_{n,k}$, with *q* tracking *x*-degree and *z* tracking $\theta $-degree.

### Theorem 1.2 (Rhoades-Wilson [Reference Rhoades and Wilson27]).

Let $k \leq n$ be positive integers, and let $N = (n-k) \cdot (k-1) + {k \choose 2}$ and $M = n-k$. We have the following facts concerning the quotient ${\mathbb {W}}_{n,k}$.

1.

*(Bidegree bound)*The bigraded piece $({\mathbb {W}}_{n,k})_{i,j}$ is zero unless $0 \leq i \leq N$ and $0 \leq j \leq M$.2.

*(Superspace Poincaré Duality)*The vector space $({\mathbb {W}}_{n,k})_{N,M} = {\mathbb {Q}}$ is 1-dimensional and spanned by $\delta _{n,k}$. For any $0 \leq i \leq N$ and $0 \leq j \leq M$, the multiplication pairing$$ \begin{align*} ({\mathbb{W}}_{n,k})_{i,j} \times ({\mathbb{W}}_{n,k})_{N-i,M-j} \longrightarrow ({\mathbb{W}}_{n,k})_{N,M} = {\mathbb {Q}} \end{align*} $$is perfect.

3.

*(Anticommuting degree zero)*The anticommuting degree zero piece of ${\mathbb {W}}_{n,k}$ is isomorphic to the quotient ring(1.11)$$ \begin{align} R_{n,k} = {\mathbb {Q}}[x_1, \dots, x_n] / \langle e_n, e_{n-1}, \dots, e_{n-k+1}, x_1^k, \dots, x_n^k \rangle , \end{align} $$where $e_d = e_d(x_1, \dots , x_n)$ is the degree

*d*elementary symmetric polynomial.4.

*(Rotational Duality)*The symmetric function ${\mathrm {grFrob}}({\mathbb {W}}_{n,k};q,z)$ admits the symmetry(1.12)$$ \begin{align} (q^N z^M) \cdot {\mathrm {grFrob}}({\mathbb{W}}_{n,k}; q^{-1}, z^{-1}) = \omega ({\mathrm {grFrob}}({\mathbb{W}}_{n,k};q,z)). \end{align} $$Here, $\omega $ is the involution on symmetric functions trading $e_n$ and $h_n$.

The rings $R_{n,k}$ appearing in Theorem 1.2 (3) were introduced by Haglund, Rhoades and Shimozono [Reference Haglund, Rhoades and Shimozono13] in their study of the Haglund-Remmel-Wilson *Delta Conjecture* [Reference Haglund, Remmel and Wilson12] (whose ‘rise formulation’ was recently proven by D’Adderio and Mellit [Reference D’Adderio and Mellit6]). Pawlowski and Rhoades [Reference Pawlowski and Rhoades21] proved that $R_{n,k}$ presents the cohomology ring $H^{\bullet }(X_{n,k}; {\mathbb {Q}})$ of the variety $X_{n,k}$ of *n*-tuples $(\ell _1, \dots , \ell _n)$ of lines in ${\mathbb {C}}^k$ which satisfy $\ell _1 + \cdots + \ell _n = {\mathbb {C}}^k$. The variety $X_{n,k}$ is smooth but not compact. Correspondingly, the Hilbert series of the ring $R_{n,k}$ is not palindromic. Theorem 1.2 (3) implies that the ‘superization’ ${\mathbb {W}}_{n,k}$ of $R_{n,k}$ satisfies a bigraded analog of Poincaré Duality. It is for this reason that our title alludes to ${\mathbb {W}}_{n,k}$ as a ‘duality module’.

Theorem 1.2 notwithstanding, the paper [Reference Rhoades and Wilson27] left many open questions about the nature of the bigraded ${\mathfrak {S}}_n$-modules ${\mathbb {W}}_{n,k}$. Indeed, the dimension of ${\mathbb {W}}_{n,k}$ was unknown. The purpose of this paper is to elucidate the structure of the duality modules ${\mathbb {W}}_{n,k}$. In order to do this, we will need the following superspace extensions of set partitions.

Definition 1.3. A *set superpartition* of $[n]$ is a set partition of $[n]$ into nonempty sets $\{ B_1, \dots , B_k \}$ in which the letters $1, \dots , n$ may be decorated with bars, and in which the minimal element $\min (B_i)$ of any block $B_i$ must be unbarred. An *ordered set superpartition* is a set superpartition $(B_1 \mid \cdots \mid B_k)$ equipped with a total order on its blocks.

As an example,

is a set superpartition of $[6]$ with three blocks. This set superpartition gives rise to $3!$ ordered set superpartitions, one of which is

where, by convention, we write elements in increasing order within blocks. Roughly speaking, barred letters will correspond algebraically to $\theta $-variables.

We define the following families of ordered set superpartitions

These sets are counted by

where ${\mathrm {Stir}}(n,k)$ is the Stirling number of the second kind counting set partitions of $[n]$ into *k* blocks.

The algebra of ${\mathbb {W}}_{n,k}$ is governed by the combinatorics of ordered superpartitions. More precisely, we prove the following.

• The ideal ${\mathrm {ann}} \, \delta _{n,k} \subseteq \Omega _n$ defining ${\mathbb {W}}_{n,k}$ has an explicit presentation (Definition 4.6) involving elementary symmetric polynomials in partial variable sets (Theorem 4.12).

• The vector space ${\mathbb {W}}_{n,k}$ has a basis indexed by ${\mathcal {OSP}}_{n,k}$ (Theorem 4.12).

• There are explicit statistics ${\mathrm {coinv}}$ and ${\mathrm {codinv}}$ on ${\mathcal {OSP}}_{n,k}$ (see Section 3), such that the bigraded Hilbert series of ${\mathbb {W}}_{n,k}$ is given by

(1.14)$$ \begin{align} {\mathrm {Hilb}}({\mathbb{W}}_{n,k}; q, z) = \sum_{r = 0}^{n-k} z^r \cdot \sum_{\sigma \in {\mathcal{OSP}}_{n,k}^{(r)}} q^{{\mathrm {coinv}}(\sigma)} = \sum_{r = 0}^{n-k} z^r \cdot \sum_{\sigma \in {\mathcal{OSP}}_{n,k}^{(r)}} q^{{\mathrm {codinv}}(\sigma)}. \end{align} $$This bigraded Hilbert series may be computed using a simple recursion (Corollary 4.13).• The $\theta $-degree pieces of ${\mathbb {W}}_{n,k}$ are built out of hook-shaped irreducibles. More precisely, if we regard ${\mathbb {W}}_{n,k}$ as a singly graded module under $\theta $-degree,

(1.15)$$ \begin{align} {\mathrm {grFrob}}({\mathbb{W}}_{n,k}; z) = \sum_{(\lambda^{(1)}, \dots, \lambda^{(k)})} z^{n - \lambda^{(1)}_1 - \cdots - \lambda^{(k)}_1} \cdot s_{\lambda^{(1)}} \cdots s_{\lambda^{(k)}}, \end{align} $$where the sum is over all*k*-tuples $(\lambda ^{(1)}, \dots , \lambda ^{(k)})$ of nonempty hook-shaped partitions which satisfy $|\lambda ^{(1)}| + \cdots + |\lambda ^{(k)}| = n$ (Corollary 5.22).• The monomial expansion of ${\mathrm {grFrob}}({\mathbb {W}}_{n,k};q,z)$ is a generating function for the statistics ${\mathrm {coinv}}$ and ${\mathrm {codinv}}$, extended to a multiset analog of ordered set superpartitions (Theorem 5.20).

Although the ${\mathrm {codinv}}$ interpretation of ${\mathrm {grFrob}}({\mathbb {W}}_{n,k};q,z)$ will implicitly describe this symmetric function as a positive sum of LLT polynomials, we do not have a combinatorial interpretation for its Schur expansion and leave this as an open problem. Our results on ${\mathbb {W}}_{n,k}$-modules are ‘superizations’ of facts about the rings $R_{n,k}$ proven in [Reference Haglund, Rhoades and Shimozono13]. Loosely speaking, ordered set partitions are replaced by ordered set superpartitions in appropriate ways. The *proofs* of these results will be significantly different from those of [Reference Haglund, Rhoades and Shimozono13] due to the anticommuting variables.

We analyse the quotient ring ${\mathbb {W}}_{n,k}$ by considering its isomorphic harmonic subspace ${\mathbb {H}}_{n,k} \subseteq \Omega _n$. This is the submodule of $\Omega _n$ generated by $\delta _{n,k}$ under the $\odot $-action:

Our analysis of ${\mathbb {H}}_{n,k}$ involves

• a new total order $\prec $ on monomials in $\Omega _n$ (see Section 4) used to describe ${\mathbb {H}}_{n,k}$ as a graded vector space, and

• a new total order $\triangleleft $ on the components of a certain direct sum decomposition $\Omega _n = \bigoplus _{p,q \geq 0} \Omega _n(p,q)$ (both depending on an auxiliary parameter

*j*) used to describe the graded ${\mathfrak {S}}_n$-structure of ${\mathbb {H}}_{n,k}$ (see Section 5).

Roughly speaking, the orders $\prec $ and $\triangleleft $ arise from the superspace intuition that a product $x_i^j \theta _i$ of an *x*-variable and the corresponding $\theta $-variable should be given a ‘negative’ exponent weight $-j$. The order $\prec $ restricts to the lexicographical term order on monomials in ${\mathbb {Q}}[x_1, \dots , x_n]$ but is not a term order on $\Omega _n$ in the sense of Gröbner theory. Indeed, the Gröbner theory of important ideals (such as the superspace coinvariant ideal) in $\Omega _n$ tends to be messier than that of analogous ideals in ${\mathbb {Q}}[x_1, \dots , x_n]$. On the other hand, we will see in Section 4 that the $\prec $-leading terms of elements in ${\mathbb {H}}_{n,k}$ correspond in a natural way to ordered set superpartitions.

It is our hope that the tools in this paper will prove useful in understanding other quotient rings involving $\Omega _n$, such as the superspace coinvariant ring. Indeed, the Fields Group has a conjecture (see [Reference Zabrocki34]) for the bigraded Frobenius image of $\Omega _n/\langle (\Omega _n)^{{\mathfrak {S}}_n}_+ \rangle $ which is equivalent (by work of [Reference Haglund, Rhoades and Shimozono13, Reference Haglund, Rhoades and Shimozono14, Reference Rhoades and Wilson27]) to

where $\{ z^{n-k} \}$ is the operator which extracts the coefficient of $z^{n-k}$. In our analysis of ${\mathbb {W}}_{n,k}$, we give an explicit generating set of its defining ideal ${\mathrm {ann}} \, \delta _{n,k}$. This gives rise (Proposition 6.4) to a side-by-side comparison of the $\theta $-degree $n-k$ pieces of $\Omega _n/\langle (\Omega _n)^{{\mathfrak {S}}_n}_+ \rangle $ and ${\mathbb {W}}_{n,k}$ as quotient modules with explicit relations. Hopefully, this similarity will assist in proving (1.17).

The rest of the paper is organised as follows. In Section 2, we give background material on superspace, ${\mathfrak {S}}_n$-modules and symmetric functions. Section 3 develops combinatorics of ordered set superpartitions necessary for the algebraic study of the ${\mathbb {W}}$-modules. Section 4 uses harmonic spaces to give a monomial basis of the modules ${\mathbb {W}}_{n,k}$ and describe their bigraded Hilbert series. Section 5 uses skewing operators and harmonics to give a combinatorial formula for the bigraded Frobenius image of the ${\mathbb {W}}$-modules. In Section 6, we conclude with some open problems.

## 2 Background

### 2.1 Alternants in superspace

Recall that if *V* is an ${\mathfrak {S}}_n$-module, a vector $v \in V$ is an *alternant* if

The superspace Vandermondes $\delta _{n,k} \in \Omega _n$ are alternants used to construct the quotient rings ${\mathbb {W}}_{n,k}$. In order to place ${\mathbb {W}}_{n,k}$ in the proper inductive context, we will need a more general family of alternants and rings.

Definition 2.1. Let $n, k, s \geq 0$ be integers. Define $\delta _{n,k,s} \in \Omega _n$ to be the element

where $\varepsilon _n = \sum _{w \in {\mathfrak {S}}_n} {\mathrm {sign}}(w) \cdot w \in {\mathbb {Q}}[{\mathfrak {S}}_n]$. Let ${\mathrm {ann}} \, \delta _{n,k,s} \subseteq \Omega _n$ be the annihilator of $\delta _{n,k,s}$, and define ${\mathbb {W}}_{n,k,s}$ to be the quotient ring

By convention, if $n < k$, then $\delta _{n,k,s} = 0$. In the special case $s = k$, we have ${\mathbb {W}}_{n,k,k} = {\mathbb {W}}_{n,k}$. We only use the ring ${\mathbb {W}}_{n,k,s}$ in the range $k \geq s$.

### 2.2 Harmonics in superspace

It will be convenient to have a model for ${\mathbb {W}}_{n,k,s}$ as a subspace rather than a quotient of $\Omega _n$. To this end, we define the *harmonic module* ${\mathbb {H}}_{n,k,s} \subseteq \Omega _n$ as follows.

Definition 2.2. Let $n, k, s \geq 0$, and consider $\Omega _n$ as a module over itself by the $\odot $-action $f \odot g = \partial f(g)$. We define ${\mathbb {H}}_{n,k,s} \subseteq \Omega _n$ to be the $\Omega _n$-submodule generated by $\delta _{n,k,s}$.

More explicitly, the harmonic module ${\mathbb {H}}_{n,k,s}$ is the smallest linear subspace of $\Omega _n$ containing $\delta _{n,k,s}$, which is closed under the action of the commuting partial derivatives $\partial /\partial x_1, \dots , \partial /\partial x_n$ as well as the anticommuting partial derivatives $\partial /\partial \theta _1, \dots , \partial /\partial \theta _n$. The subspace ${\mathbb {H}}_{n,k,s}$ is a bigraded ${\mathfrak {S}}_n$-module.

We have a natural inclusion map ${\mathbb {H}}_{n,k,s} \hookrightarrow \Omega _n$. The composition

of this inclusion with the canonical projection of $\Omega _n$ onto ${\mathbb {W}}_{n,k,s}$ is an isomorphism of bigraded ${\mathfrak {S}}_n$-modules. We make use of ${\mathbb {H}}_{n,k,s}$ when we need to consider superspace elements in $\Omega _n$ rather than cosets in ${\mathbb {W}}_{n,k,s}$.

### 2.3 Symmetric functions and ${\mathfrak {S}}_n$-modules

Throughout this paper, we use the following standard *q*-analogs of numbers, factorials and binomial coefficients:

A *partition* $\lambda $ of $ n$ is a weakly decreasing sequence $\lambda = (\lambda _1 \geq \cdots \geq \lambda _k)$ of positive integers which sum to *n*. We write $\lambda \vdash n$ to mean that $\lambda $ is a partition of *n*.

Let $\Lambda = \bigoplus _{n \geq 0} \Lambda _n$ be the ring of symmetric functions in an infinite variable set ${\mathbf {x}} = (x_1, x_2, \dots )$ over the ground field ${\mathbb {Q}}(q,z)$. Bases of the $n^{th}$ graded piece $\Lambda _n$ of this ring are indexed by partitions $\lambda \vdash n$. We let

be the *monomial, elementary, homogeneous* and *Schur* bases of $\Lambda _n$. Given two partitions $\lambda , \mu $ with $\lambda _i \geq \mu _i$ for all *i*, we let $s_{\lambda /\mu }$ be the corresponding *skew Schur function*.

A formal power series *F* in the variable set ${\mathbf {x}} = (x_1, x_2, \dots )$ of bounded degree is *quasisymmetric* if the coefficient of $x_1^{a_1} \cdots x_n^{a_n}$ equals the coefficient of $x_{i_1}^{a_1} \cdots x_{i_n}^{a_n}$ for any strictly increasing sequence $i_1 < \cdots < i_n$ of indices. Given a subset $S \subseteq [n-1]$, the *fundamental quasisymmetric function* of degree *n* is

We will encounter the formal power series $F_{S,n}$ exclusively in the case where *S* is the *inverse descent set* of a permutation $w \in {\mathfrak {S}}_n$. This is the set

We let $\langle -, - \rangle $ be the *Hall inner product* on $\Lambda _n$ obtained by declaring the Schur functions $s_{\lambda }$ to be orthonormal. For any $F \in \Lambda $, we have a ‘skewing’ operator $F^{\perp }: \Lambda \rightarrow \Lambda $ characterised by

for all $G, H \in \Lambda $. We will make use of the following fact.

Lemma 2.3. Let $F, G \in \Lambda $ be two homogeneous symmetric functions of positive degree. The following are equivalent.

1. We have $F = G$.

2. We have $h_j^{\perp } F = h_j^{\perp } G$ for all $j \geq 1$.

3. We have $e_j^{\perp } F = e_j^{\perp } G$ for all $j \geq 1$.

Lemma 2.3 follows from the fact that either of the sets $\{e_1, e_2, \dots \}$ or $\{h_1, h_2, \dots \}$ are algebraically independent generating sets of the ring $\Lambda $ of symmetric functions.

Irreducible representations of ${\mathfrak {S}}_n$ are in bijective correspondence with partitions $\lambda $ of *n*. If $\lambda \vdash n$ is a partition, let $S^{\lambda }$ be the corresponding irreducible ${\mathfrak {S}}_n$-module. If *V* is any finite-dimensional ${\mathfrak {S}}_n$-module, there are unique multiplicities $c_{\lambda } \geq 0$, such that $V \cong \bigoplus _{\lambda \vdash n} c_{\lambda } S^{\lambda }$. The *Frobenius image* of *V* is the symmetric function

obtained by replacing each irreducible $S^{\lambda }$ with the corresponding Schur function $s_{\lambda }$.

Given two positive integers $n, m$, we have the corresponding *parabolic subgroup* ${\mathfrak {S}}_n \times {\mathfrak {S}}_m \subseteq {\mathfrak {S}}_{n+m}$ obtained by permuting the first *n* letters and the last *m* letters in $[n+m]$ separately. If *V* is an ${\mathfrak {S}}_n$-module and *W* is an ${\mathfrak {S}}_m$-module, their *induction product* $V \circ W$ is the ${\mathfrak {S}}_{n+m}$-module given by

Induction product and Frobenius image are related in that

Frobenius images interact with the skewing operators $h_j^{\perp }$ and $e_j^{\perp }$ in the following way. Let *V* be an ${\mathfrak {S}}_n$-module, let $1 \leq j \leq n$ and consider the parabolic subgroup ${\mathfrak {S}}_j \times {\mathfrak {S}}_{n-j}$ of ${\mathfrak {S}}_n$. We have the group algebra elements $\eta _j, \varepsilon _j \in {\mathbb {Q}}[{\mathfrak {S}}_j]$

which symmetrise and antisymmetrise in the **first** *j* letters, respectively. Since $\eta _j$ and $\varepsilon _j$ commute with permutations in the second parabolic factor ${\mathfrak {S}}_{n-j}$, the vector spaces

are naturally ${\mathfrak {S}}_{n-j}$-modules. The Frobenius images of these modules are as follows.

Lemma 2.4. We have ${\mathrm {Frob}}(\eta _j V) = h_j^{\perp } {\mathrm {Frob}}(V)$ and ${\mathrm {Frob}}(\varepsilon _j V) = e_j^{\perp } {\mathrm {Frob}}(V)$.

The proof of Lemma 2.4, which we omit, uses Frobenius reciprocity. Lemma 2.4 may be generalised by considering the image of *V* under $\sum _{w \in {\mathfrak {S}}_j} \chi ^{\mu }(w) \cdot w \in {\mathbb {Q}}[{\mathfrak {S}}_j]$, where $\mu \vdash j$ is any partition and $\chi ^{\mu }: {\mathfrak {S}}_j \rightarrow {\mathbb {C}}$ is the irreducible character; the effect on Frobenius images is the operator $s_{\mu }^{\perp }$.

In this paper, we will consider (bi)graded vector spaces and modules. If $V = \bigoplus _{i \geq 0} V_i$ is a graded vector space with each piece $V_i$ finite-dimensional, recall that its *Hilbert series* is given by

Similarly, if $V = \bigoplus _{i,j \geq 0} V_{i,j}$ is a bigraded vector space, we have the *bigraded Hilbert series*

If $V = \bigoplus _{i \geq 0} V_i$ is a graded ${\mathfrak {S}}_n$-module, its *graded Frobenius image* is

Extending this, if $V = \bigoplus _{i,j \geq 0} V_{i,j}$ is a bigraded ${\mathfrak {S}}_n$-module, its *bigraded Frobenius image* is

### 2.4 Ordered set superpartitions

We will show that the duality modules ${\mathbb {W}}_{n,k}$ are governed by the combinatorics of ordered set superpartitions in ${\mathcal {OSP}}_{n,k}$. The more general modules ${\mathbb {W}}_{n,k,s}$ of Definition 2.1 which we will use to inductively describe the ${\mathbb {W}}_{n,k}$ are controlled by the following more general combinatorial objects.

Definition 2.5. For $n, k, s \geq 0$, we let ${\mathcal {OSP}}_{n,k,s}$ be the family of *k*-tuples $(B_1 \mid \cdots \mid B_k)$ of sets of positive integers, such that

• we have the disjoint union decomposition $[n] = B_1 \sqcup \cdots \sqcup B_k$,

• the first

*s*sets $B_1, \dots , B_s$ are nonempty,• the elements of $B_1, \dots , B_k$ may be barred or unbarred,

• the minimal elements $\min B_1, \dots , \min B_s$ of the first

*s*sets are unbarred.

We denote by ${\mathcal {OSP}}_{n,k,s}^{(r)} \subseteq {\mathcal {OSP}}_{n,k,s}$ the subfamily of $\sigma \in {\mathcal {OSP}}_{n,k,s}$ with *r* barred elements.

Note that, when $s=k$, ${\mathcal {OSP}}_{n,k} = {\mathcal {OSP}}_{n,k,s}$. We refer to elements $\sigma = (B_1 \mid \cdots \mid B_k) \in {\mathcal {OSP}}_{n,k,s}$ as *ordered set superpartitions*, despite the fact that any of the last $k-s$ sets $B_{s+1}, \dots , B_k$ in $\sigma $ could be empty and that the minimal elements of $B_{s+1}, \dots , B_k$ (if they exist) may be barred.

## 3 Ordered set superpartitions

### 3.1 The statistics ${\mathrm {coinv}}$ and ${\mathrm {codinv}}$

In this section, we define two statistics on ${\mathcal {OSP}}_{n,k,s}$. The first of these is an extension of the classical inversion statistic (or rather, its complement) on permutations in ${\mathfrak {S}}_n$. Given $\pi = \pi _1 \dots \pi _n \in {\mathfrak {S}}_n$, its *coinversion code* is the sequence $(c_1, \dots , c_n)$, where $c_i$ is the number of entries in the set $\{i+1, i+2, \dots , n \}$ which appear to the right of *i* in $\pi $. The *coinversion number* of $\pi $ is the sum ${\mathrm {coinv}}(\pi ) = c_1 + \cdots + c_n$. We generalise these concepts as follows.

Definition 3.1. Let $\sigma = (B_1 \mid \cdots \mid B_k) \in {\mathcal {OSP}}_{n,k,s}$ be an ordered set superpartition. The *coinversion code* is the length *n* sequence ${\mathrm {code}}(\sigma ) = (c_1, \dots , c_n)$ over the alphabet $\{0, 1, 2, \dots , \bar {0}, \bar {1}, \bar {2}, \dots \}$ whose $a^{th}$ entry $c_a$ is defined as follows. Suppose that *a* lies in the $i^{th}$ block $B_i$ of $\sigma $.

• The entry $c_a$ is barred if and only if

*a*is barred in $\sigma $.• If $a = \min B_i$ and $i \leq s$, then

$$ \begin{align*} c_a = \left| \{ i+1 \leq j \leq s \,:\, \min B_j> a \} \right|. \end{align*} $$• If

*a*is barred, then$$ \begin{align*} c_a = \begin{cases} | \{ 1 \leq j \leq i-1 \,:\, \min B_j < a \} | & \text{if } i \leq s, \\ | \{ 1 \leq j \leq s \,:\, \min B_j < a \} | + (i-s-1) & \text{if } i> s. \end{cases} \end{align*} $$• Otherwise, we set

$$ \begin{align*} c_a = | \{ i+1 \leq j \leq s \,:\, \min B_j> a \} | + (i-1). \end{align*} $$

The *coinversion number* of $\sigma $ is the sum

of the entries $(c_1, \dots , c_n)$ in ${\mathrm {code}}(\sigma )$.

For example, consider the ordered set superpartition

The coinversion code of $\sigma $ is given by

so that

Bars on the entries $c_1, \dots , c_n$ are ignored when calculating ${\mathrm {coinv}}(\sigma ) = c_1 + \cdots + c_n$.

The coinversion code $(c_1, \dots , c_n)$ can be visualised by considering the *column diagram* notation for ordered set superpartitions. Given $\sigma = (B_1 \mid \cdots \mid B_k) \in {\mathcal {OSP}}_{n,k,s}$, we draw the entries of $B_i$ in the $i^{th}$ column. The entries of $B_i$ fill the $i^{th}$ column according to the following rules.

• For $1 \leq i \leq k$, the barred entries of $B_i$ start at height 1 and fill up in increasing order.

• For $1 \leq i \leq s$, the unbarred entries of $B_i$ start at height 0 and fill down in increasing order.

• For $s+1 \leq i \leq k$, the unbarred entries of $B_i$ start at height $-1$ and fill down in increasing order; we also place a $\bullet $ at height 0 in these columns.

In our example, the column diagram of

is given by

where the column indices are shown below and the heights of entries are shown on the left. The three $\bullet $’s at the height 0 level correspond to the fact that the blocks $B_3, B_4, B_5$ are allowed to be empty for $\sigma \in {\mathcal {OSP}}_{9,5,2}$. Given $\sigma \in {\mathcal {OSP}}_{n,k,s}$, we have the following column diagram interpretation of the $a^{th}$ letter $c_a$ of ${\mathrm {code}}(\sigma ) = (c_1, \dots , c_n)$.

• If

*a*appears at height 0, then $c_a$ counts the number of height zero entries to the right of*a*which are $> a$.• If

*a*appears at negative height in column*i*, then $c_a$ is $i-1$, plus the number of height 0 entries to the right of*a*which are $> a$.• If

*a*appears at positive height, then $c_a$ counts the number of height 0 entries to the left of*a*which are $< a$, plus the number of $\bullet $’s to the left of*a*.

We will see that codes $(c_1, \dots , c_n)$ of elements $\sigma \in {\mathcal {OSP}}_{n,k,s}$ correspond to a monomial basis of the quotient ring ${\mathbb {W}}_{n,k,s}$. In order to obtain the bigraded Frobenius image of ${\mathbb {W}}_{n,k,s}$ in terms of ${\mathrm {coinv}}$, we use diagrams to define a formal power series as follows.

The *reading word* of $\sigma \in {\mathcal {OSP}}_{n,k,s}$, denoted ${\mathrm {read}}(\sigma )$ is the permutation in ${\mathfrak {S}}_n$ obtained by reading the column diagram of $\sigma $ from top to bottom, and, within each row, **from right to left** (ignoring any bars on letters). If $\sigma $ is our example ordered set partition above, we have

Definition 3.2. Let $n, k, s \geq 0$ be integers, and let $0 \leq r \leq n-s$. We define a quasisymmetric function $C^{(r)}_{n,k,s}({\mathbf {x}};q)$ by the formula

Also define a quasisymmetric function $C_{n,k,s}({\mathbf {x}};q,z)$ by

We may avoid the use of the *F*’s in the definition of the *C*-functions by allowing repeated entries in our column diagrams. Let ${\mathcal {CT}}_{n,k,s}$ be the family of column tableaux which are all fillings of finite subsets of the infinite strip $[k] \times \infty $ with the alphabet $\{ \bullet , 1, 2, \dots , \bar {1}, \bar {2}, \dots \}$, such that

• The height 0 row is filled with a sequence of

*s*unbarred numbers, followed by a sequence of $k-s \bullet $’s.• The filled cells form a contiguous sequence within each column.

• Entries at positive height are barred, while entries at negative height are unbarred.

• Unbarred numbers weakly increase going down, and barred numbers strictly increase going up.

• An unbarred number at height 0 is strictly smaller than any barred number above it.

An example column tableau $\tau \in {\mathcal {CT}}_{13, 5, 2}$ is shown below.

The coinversion number ${\mathrm {coinv}}(\tau )$ extends naturally to column tableaux $\tau \in {\mathcal {CT}}_{n,k,s}$: given an entry *a* in such a tableau, we may compute its contribution $c_a$ to ${\mathrm {coinv}}$ as before, and sum over all entries. For example, the column tableau depicted above has coinversion number

where the entries in the sum on the left are processed in reading order, that is the first value in the sum is 3 because the $\bar {4}$ in column 4 contributes 3 to the coinversion number. Let ${\mathbf {x}}^{\tau } = x_1^{\alpha _1} x_2^{\alpha _2} \cdots $, where $\alpha _i$ is the number of $i's$ in $\tau $. In our example, we have ${\mathbf {x}}^{\tau } = x_1^3 x_2 x_3^3 x_4^3 x_5 x_6 x_7$. The exponent sequence $\alpha = (\alpha _1, \alpha _2, \dots )$ of ${\mathbf {x}}^{\tau }$ is called the *content* of $\tau $. Let ${\mathcal {CT}}_{n,k,s}^{(r)} \subseteq {\mathcal {CT}}_{n,k,s}$ be the subfamily of column tableaux with *r* barred letters.

Observation 3.3. The formal power series $C_{n,k,s}^{(r)}({\mathbf {x}};q)$ and $C_{n,k,s}({\mathbf {x}};q,z)$ are given by

This observation follows from the definition of the fundamental quasisymmetric functions. The formulas for the *C*-functions in Observation 3.3 are more aesthetic but less efficient than those in Definition 3.2. It will turn out that $C_{n,k,s}({\mathbf {x}};q,z)$ is the bigraded Frobenius image ${\mathrm {grFrob}}({\mathbb {W}}_{n,k,s};q,z)$. For reasons related to skewing recursions, the refinement $C^{(r)}_{n,k,s}({\mathbf {x}};q)$ will be convenient to consider. At this point, it is not clear that the *C*-functions are even symmetric. Their symmetry (and Schur positivity) will follow from an alternative description in terms of another statistic on ${\mathcal {OSP}}_{n,k,s}$.

Let $\sigma \in {\mathcal {OSP}}_{n,k,s}$. We augment the column diagram of $\sigma $ by placing infinitely many $+ \infty $’s below the entries in every column (we drop the $+$ in diagrams for the sake of compactness). Furthermore, we regard every $\bullet $ in the column diagram as being filled with a 0. Ignoring bars, a pair of entries $a < b$ form a *diagonal coinversion pair* if

•

*a*appears to the left of and at the same height as*b*, or•

*a*appears to the right of and at height one less than*b*.

Schematically, these conditions have the form

for $a < b$.

Definition 3.4. For $\sigma \in {\mathcal {OSP}}_{n,k,s}$, the *diagonal coinversion number* ${\mathrm {codinv}}(\sigma )$ is the total number of diagonal coinversion pairs in the augmented column diagram of $\sigma $.

When $n=k=s$, ${\mathrm {codinv}}$ and ${\mathrm {coinv}}$ are identical. Although these statistics clearly differ in general, it will turn out that ${\mathrm {codinv}}$ is equidistributed with ${\mathrm {coinv}}$ on ${\mathcal {OSP}}_{n,k,s}$ and even on the subsets ${\mathcal {OSP}}_{n,k,s}^{(r)}$ obtained by restricting to *r* barred letters. In analogy with the *C*-functions, we define a quasisymmetric function attached to ${\mathrm {codinv}}$.

Definition 3.5. Let $n, k, s \geq 0$ be integers, and let $0 \leq r \leq n-s$. Define a quasisymmetric function $D^{(r)}_{n,k,s}({\mathbf {x}};q)$ by the formula

Also define

Like the *C*-functions, the *D*-functions may be expressed in terms of infinite sums over column tableaux.

Observation 3.6. The formal power series $D_{n,k,s}^{(r)}({\mathbf {x}};q)$ and $D_{n,k,s}({\mathbf {x}};q,z)$ are given by

We use the theory of LLT polynomials to show that the *D*-functions are symmetric and Schur positive.

Proposition 3.7. The quasisymmetric functions $D_{n,k,s}^{(r)}({\mathbf {x}};q)$ and $D_{n,k,s}({\mathbf {x}};q,z)$ are symmetric and Schur positive.

Proof. We prove this fact by writing $D_{n,k,s}({\mathbf {x}}; q, z)$ as a positive linear combination of *LLT polynomials* [Reference Lascoux, Leclerc and Thibon18]. We use the version of LLT polynomials employed by Haglund, Haiman and Loehr [Reference Haglund, Haiman and Loehr11].

A *skew diagram* is a set of cells in the first quadrant given by $\mu / \nu $ for some partitions $\mu \supseteq \nu $. Given a tuple $ {\boldsymbol\lambda } = (\lambda ^{(1)}, \lambda ^{(2)}, \ldots , \lambda ^{(l)})$ of skew diagrams, the LLT polynomial indexed by ${\boldsymbol\lambda }$ is

where the sum is over all semistandard fillings of the skew diagrams of $ {\boldsymbol\lambda }$, ${\mathbf {x}}^T$ is the product, where $x_i$ appears as many times as *i* appears in the filling *T*, and ${\mathrm {inv}}(T)$ is the following statistic. Given a cell *u* with coordinates $(x, y)$ in one of the skew shapes $\lambda ^{(i)}$, where the leftmost cell at height 0 has coordinates (0,0), the *content* of *u* is $c(u) = x - y$. Then ${\mathrm {inv}}(T)$ is the number of pairs of cells $u \in \lambda ^{(i)}$, $v \in \lambda ^{(j)}$ with $i < j$, such that

• $c(u) = c(v)$ and $T(u)> T(v)$, or

• $c(u) + 1 = c(v)$ and $T(u) < T(v)$.

LLT polynomials are Schur-positive symmetric functions [Reference Grojnowski and Haiman10, Reference Haglund, Haiman and Loehr11, Reference Lascoux, Leclerc and Thibon18]. We claim that we can decompose $D_{n,k,s}^{(r)}({\mathbf {x}};q)$ into LLT polynomials:

where $\mathrm {stat}$ is a fixed statistic depending on $ {\boldsymbol\lambda }$ and the sum is over all tuples of skew diagrams $ {\boldsymbol\lambda } = (\lambda ^{(1)}, \ldots , \lambda ^{(k)})$ satisfying

• if $i \leq k-s$, then $\lambda ^{(i)} = \mu ^{(i)} / (1)$ for a hook shape $\mu ^{(i)}$,

• if $i> k-s$, then $\lambda ^{(i)}$ is a single nonempty hook shape,

• $\sum _{i=1}^{k} |\lambda ^{(i)}| = n$ and

• $ {\boldsymbol\lambda }$ has

*r*total cells of negative content.

We define nonnegative integer sequences $\alpha , \beta \in \mathbb {N}^{k}$ so that

• $\lambda ^{(i)} = (\alpha _i + 1, 1^{\beta _i}) / (1)$ for $i \leq k-s$ and

• $\lambda ^{(i)} = (\alpha _i + 1, 1^{\beta _i})$ for $i> k-s$.

Given a specific semistandard filling *T* that contributes to $\mathrm {LLT}_{{\boldsymbol\lambda }}({\mathbf {x}}; q)$ for such a $ {\boldsymbol\lambda }$, we will create a column tableau $\tau \in {\mathcal {CT}}_{n,k,s}^{(r)}$, such that $\tau $ will have exactly $\alpha _i$ entries at negative height and $\beta _i$ entries at positive height in column $k-i+1$. The sequences $\alpha $ and $\beta $ will completely determine how many diagonal coinversions involving an $\infty $ or a $\bullet $, respectively, appear in $\tau $. In Figure 1, there are six such diagonal coinversions involving an $\infty $ and seven involving a $\bullet $. In general, this number is

where $\chi (p)$ is 1 if *p* is true and $0$ if *p* is false. Note that $\mathrm {stat}( {\boldsymbol\lambda })$ can be computed from $\alpha $ and $\beta $ only.

Now, given a specific semistandard filling *T* of $ {\boldsymbol\lambda }$, we map the unique entry in $\lambda ^{(i)}$ with content *j* to column $k-i+1$ and row $-j$ in $\tau $. Every pair which contributes to ${\mathrm {inv}}(T)$ now contributes a diagonal coinversion between integers (not $\bullet $ or $\infty $) in $\tau $. In Figure 1, there are 10 such diagonal coinversions and the filling *T* has ${\mathrm {inv}}(T) = 10$. Since this map is bijective, we have the desired result.

### 3.2 Equality of the *C*- and *D*-functions

Our first main result states that the *C*-functions and *D*-functions coincide. This given, Proposition 3.7 implies that the *C*-functions are symmetric and Schur positive. We know of no direct proof of either of these facts.

Theorem 3.8. For any integers $n,k,s \geq 0$ with $k \geq s$ and any $0 \leq r \leq n-s$, we have the equality of formal power series

Consequently, we have

Our proof of Theorem 3.8 is combinatorial and uses the column tableau forms of the formal power series $C_{n,k,s}^{(r)}({\mathbf {x}};q)$ and $D_{n,k,s}^{(r)}({\mathbf {x}};q)$ in Observations 3.3 and 3.6. The idea is to consider building up a general column tableau $\tau \in {\mathcal {CT}}_{n,k,s}^{(r)}$ from the column tableau consisting of $k-s$ empty columns by successively adding larger entries and showing that the statistics ${\mathrm {coinv}}$ and ${\mathrm {codinv}}$ satisfy the same recursions.

Proof. Let $\tau $ be a column tableau with *k* columns and $k-s \bullet $’s whose entries are $< N$. We consider building a larger column tableau involving *N*’s from $\tau $ by the following three-step process.

1. Placing $\bar {N}$’s on top of some subset of the

*k*columns of $\tau $.2. Placing

*N*’s at height 0 between and on either side of the*s*columns of $\tau $ without a $\bullet $.3. Placing

*N*’s at negative heights below some multiset of columns of the resulting figure.

We track the behavior of ${\mathrm {coinv}}$ and ${\mathrm {codinv}}$ as we perform this procedure, starting by placing the $\bar {N}$’s on top of columns.

Placing a $\bar {N}$ on top of column *i* of $\tau $ increases the statistic ${\mathrm {coinv}}$ by $i-1$. We reflect this fact by giving column *i* the *barred* ${\mathrm {coinv}}$ *label* of $i-1$. The barred ${\mathrm {coinv}}$ labels of the column tableau are shown in bold and barred below.

The barred ${\mathrm {coinv}}$ labels give rise to a bijection

by letting $\iota ^{{\mathrm {coinv}}}_{\bar {N}}(\tau ,S)$ be the tableau $\tau '$ obtained by placing a $\bar {N}$ on top of every column with barred ${\mathrm {coinv}}$ label in *S*. If $\iota ^{{\mathrm {coinv}}}_{\bar {N}}: (\tau ,S) \mapsto \tau '$, then ${\mathrm {coinv}}(\tau ') = {\mathrm {coinv}}(\tau ) + \sum _{i \in S} i$.

Next, we consider the effect of $\bar {N}$ insertion on ${\mathrm {codinv}}$. We bijectively label the *k* columns of $\tau $ with the *k barred* ${\mathrm {codinv}}$ *labels* $0, 1, \dots , k-1$ (in that order) in descending order of maximal height and, within columns of the same maximal height, proceeding from right to left. The barred ${\mathrm {codinv}}$ labels in our example are shown below.

The barred ${\mathrm {codinv}}$ labels give a bijection

where $\iota ^{{\mathrm {codinv}}}_{\bar {N}}(\tau ,S)$ is obtained from $\tau $ by placing a $\bar {N}$ on top of every column with barred ${\mathrm {codinv}}$ label indexed by *S*. If $\iota ^{{\mathrm {codinv}}}_{\bar {N}}: (\tau ,S) \mapsto \tau '$, then ${\mathrm {codinv}}(\tau ') = {\mathrm {codinv}}(\tau ) + \sum _{i \in S} i$.

We move on to Step 2 of our insertion procedure: creating new columns by placing *N*’s at height 0. A single ‘height 0 insertion map’ $\iota _0$ of this kind on column tableaux has the same effect on ${\mathrm {coinv}}$ and ${\mathrm {codinv}}$.

If $\tau $ is a column tableau with *s* nonbullet letters at height zero, *N*’s can be placed (with repetition) in any of the $s+1$ places between and on either side of these nonbullet letters. This gives rise to a bijection

where $\iota _0(\tau ',S)$ is obtained from $\tau '$ by inserting $m_i$ copies of *N* after column *i* of $\tau $, where $m_i$ is the multiplicity of *i* in *S*. Suppose $\iota _0(\tau ',S) = \tau "$. The number *K* of columns of $\tau "$ is related to the number *k* of columns of $\tau '$ by $K = k + |S|$. Furthermore, if there are *b* unbarred entries in $\tau '$ of negative height, we have

In other words, the height zero insertion map $\iota _0$ has the same effect on ${\mathrm {coinv}}$ and ${\mathrm {codinv}}$.

Finally, let $\tau "$ be a column tableau with *K* columns with entries $\leq N$ in which there are no *N*’s with negative height. To perform Step 3 of our insertion process, we insert *N*’s at the bottom of some multiset of columns of $\tau "$. We track the effect on ${\mathrm {coinv}}$ and ${\mathrm {codinv}}$ as before.

We label the columns of $\tau "$ from left-to-right with the *unbarred* ${\mathrm {coinv}}$ *labels* $0,1, \dots , K$. An example of this labeling is shown below in bold. Since we add unbarred letters on the bottom of a tableau, we show the unbarred labels there as well.

The unbarred ${\mathrm {coinv}}$ labels give a bijection

where $\iota ^{{\mathrm {coinv}}}_N(\tau ",S)$ is obtained by placing $m_i$ copies of *N* below the column with label *i*, where $m_i$ is the multiplicity of *i* in *S*. If $\iota ^{{\mathrm {coinv}}}_N: (\tau ",S) \mapsto \tau "'$, then ${\mathrm {coinv}}(\tau "') = {\mathrm {coinv}}(\tau ") + \sum _{i \in S} i$.

The effect of inserting *N*’s at negative height on ${\mathrm {codinv}}$ may be described as follows. We label the columns of $\tau $ bijectively with the *unbarred* ${\mathrm {codinv}}$ *labels* $0,1,\dots ,k-1$ (in that order) starting at columns of lesser maximal depth and, within columns of the same maximal depth, proceeding from right to left. The unbarred ${\mathrm {codinv}}$ labels in our example as shown below.

The unbarred ${\mathrm {codinv}}$ labels give a bijection

as follows. Given $(\tau ",S)$, we process the entries of *S* **in weakly increasing order** and, given an entry *i*, we place an *N* at the bottom of the column with unbarred ${\mathrm {codinv}}$ label *i*. Recalling that we consider every unfilled cell below a column to have the label $+ \infty $ for the purpose of calculating ${\mathrm {codinv}}$, we see that if $\iota ^{{\mathrm {codinv}}}_N: (\tau ",S) \mapsto \tau "'$, then ${\mathrm {codinv}}(\tau "') = {\mathrm {codinv}}(\tau ") + \sum _{i \in S} i$. In summary, the $\iota $-maps show that the *C*-functions and *D*-functions satisfy the same recursion, which establishes the theorem.

Although the *D*-functions are more directly seen to be symmetric and Schur positive, in order to describe a recursive formula for the action of $h_j^{\perp }$, it will be more convenient to use the *C*-functions instead (Lemma 3.14). This formula will involve the more refined $C_{n,k,s}^{(r)}({\mathbf {x}};q)$ rather than their coarsened versions $C_{n,k,s}({\mathbf {x}};q,z)$.

### 3.3 Substaircase shuffles

Given $\sigma \in {\mathcal {OSP}}_{n,k,s}$, ${\mathrm {code}}(\sigma ) = (c_1, \dots , c_n)$ is a length *n* sequence over the alphabet $\{0,1,2,\dots ,\bar {0},\bar {1},\bar {2},\dots \}$. In this subsection, we show that $\sigma \mapsto {\mathrm {code}}(\sigma )$ is injective and characterise its image. We begin by defining a partial order on sequences of (potentially barred) nonnegative integers.

Definition 3.9. $(c_1, \dots , c_n) \leq (b_1, \dots , b_n)$ if, for every *i*, $c_i \leq b_i$ in value and that $c_i$ is barred if and only if $b_i$ is barred.

The image of the classical coinversion code map on permutations in ${\mathfrak {S}}_n$ is given by words $(c_1, \dots , c_n)$ which are $\leq $ the ‘staircase’ word $(n-1, n-2, \dots , 1, 0)$. For ordered set partitions of $[n]$ into *k* blocks with no barred letters, this result was generalised in [Reference Rhoades and Wilson26], where the appropriate notion of ‘staircase’ is given by a shuffle of two sequences. We extend these definitions to barred letters as follows.

Recall that a *shuffle* of two sequences $(a_1, \dots , a_r)$ and $(b_1, \dots , b_s)$ is an interleaving $(c_1, \dots , c_{r+s})$ of these sequences which preserves the relative order of the *a*’s and the *b*’s. The following collection ${\mathcal {SS}}^{(r)}_{n,k,s}$ of words will turn out to be the image of the map ${\mathrm {code}}$ on ${\mathcal {OSP}}_{n,k,s}^{(r)}$.

Definition 3.10. Let $n, k, s \geq 0$ be integers. For $0 \leq r \leq n-s$, a *staircase* with *r* barred letters is a length *n* word obtained by shuffling

where $r_0 + r_1 + \cdots + r_s = r$. If $k=s$, we insist that $r_0 = 0$ in the above expression. Let ${\mathcal {SS}}^{(r)}_{n,k,s}$ be the family of words $(c_1, \dots , c_n)$ which are $\leq $ some staircase $(b_1, \dots , b_n)$. We also let

and refer to words in ${\mathcal {SS}}_{n,k,s}$ as *substaircase*.

Definition 3.10 implies that

We give an example to clarify these concepts.

Example 3.11. Consider the case $n = 5, k = 3, s = 2$ and $r = 2$. The staircases with *r* barred letters are the shuffles of any of the six words

with the single-letter word $(2)$. For example, if we shuffle $(2)$ into the second sequence from the left, we get the five staircases

The leftmost sequence displayed above contributes

to ${\mathcal {SS}}_{5,3,2}^{(2)}$. These are the 12 sequences which have the same bar pattern as, and are componentwise $\leq $ to, the staircase $(2, \bar {0}, 1, \bar {1}, 0)$.

The previous example shows that applying Definition 3.10 to obtain the set of words in ${\mathcal {SS}}_{n,k,s}$ can be involved. The following lemma gives a simple recursive definition of substaircase sequences of length *n* in terms of substaircase sequences of length $n-1$. This recursion gives an efficient way to calculate ${\mathcal {SS}}_{n,k,s}$ and will be useful in our algebraic analysis of ${\mathbb {W}}_{n,k,s}$ in Section 4. It will turn out that words in ${\mathcal {SS}}_{n,k,s}$ index a monomial basis of ${\mathbb {W}}_{n,k,s}$.

Lemma 3.12. Let $n, k, s \geq 0$ be integers with $k \geq s$, and let $0 \leq r \leq n-s$. The set ${\mathcal {SS}}_{n,k,s}^{(r)}$ has the disjoint union decomposition

and the set ${\mathcal {SS}}_{n,k,s}$ has the disjoint union decomposition

Proof. The second disjoint union decomposition for ${\mathcal {SS}}_{n,k,s}$ follows from the first disjoint union decomposition for ${\mathcal {SS}}_{n,k,s}^{(r)}$ by taking the (disjoint) union over all *r*, so we focus on the first decomposition.

Given $(c_1, c_2, \dots , c_n) \in {\mathcal {SS}}_{n,k,s}^{(r)}$, there exists some word $(b_1, b_2, \dots , b_n)$ obtained by shuffling

where $r_0 + r_1 + \cdots + r_s = r$ with the constant sequence $(k-1)^{n-r-s}$, such that $(c_1, c_2, \dots , c_n)$ has the same bar pattern as $(b_1, b_2, \dots , b_n)$ and $c_i \leq b_i$ for all *i*. There are three possibilities for the first entry $c_1$.

• If $c_1 = \bar {a}$ is barred, then we must have $r_0> 0$ and $b_1 = \overline {k-s-1}$. It follows that $0 \leq a \leq k-s-1$. Furthermore, the word $(b_2, \dots , b_n)$ is a shuffle of

$$ \begin{align*} ( \overline{k-s-1} )^{r_0-1} (s-1) ( \overline{k-s} )^{r_1} (s-2) ( \overline{k-s+1} )^{r_2} (s-3) \cdots 0 ( \overline{k-1} )^{r_s} \quad \text{and} \quad (k-1)^{n-r-s}. \end{align*} $$This implies that $(c_2, \dots , c_n) \in {\mathcal {SS}}_{n-1,k,s}^{(r-1)}$. Conversely, given $(c_2, \dots , c_n) \in {\mathcal {SS}}_{n-1,k,s}^{(r-1)}$ and $0 \leq a \leq k-s-1$, we see that $(\bar {a}, c_2, \dots , c_n) \in {\mathcal {SS}}_{n,k,s}^{(r)}$ by prepending the letter $\overline {k-s-1}$ to any $(n-1,k,s)$-staircase $(b_2, \dots , b_n) \geq (c_2, \dots , c_n)$.• If $c_1 = a$ is unbarred with $0 \leq a \leq s - 1$, the first letter $b_1$ of $(b_1, b_2, \dots , b_n)$ must also be unbarred and $r_0 = 0$. This means that $b_1 = s-1$ or $k-1$. If $k> s$ and $b_1 = k-1$, we may interchange the $b_1$ with the unique occurrence of $s-1$ in $(b_1, b_2, \dots , b_n)$ to get a new staircase $(b^{\prime }_1, b^{\prime }_2, \dots , b^{\prime }_n)$ which shares the color pattern of $(c_1, c_2, \dots , c_n)$ and satisfies $c_i \leq b^{\prime }_i$ for all

*i*. We may, therefore, assume that $b_1 = s-1$. This means that the sequence $(b_2, \dots , b_n)$ is a shuffle of the words$$ \begin{align*} ( \overline{k-s} )^{r_1} (s-2) ( \overline{k-s+1} )^{r_2} (s-3) \cdots 0 ( \overline{k-1} )^{r_s} \quad \text{and} \quad (k-1)^{n-r-s} \end{align*} $$and $r_1 + r_2 + \cdots + r_s = r$. Therefore, we have $(c_2, \dots , c_n) \in {\mathcal {SS}}_{n-1,k,s-1}^{(r)}$. Conversely, given $(c_2, \dots , c_n) \in {\mathcal {SS}}_{n-1,k,s-1}^{(r)}$, one sees that $(a, c_2, \dots , c_n) \in {\mathcal {SS}}_{n,k,s}^{(r)}$ by prepending an $s-1$ to any $(n-1,k,s-1)$-staircase $(b_2, \dots , b_n) \geq (c_2, \dots , c_n)$.• Finally, if $c_1 = a$ is unbarred and $c_1 \geq s$, the first letter $b_1$ of $(b_1, b_2, \dots , b_n)$ must be unbarred and $\geq s$. This implies that $b_1 = k-1$, so that $s \leq a < k-1$. Furthermore, the sequence $(b_2, \dots , b_n)$ is a shuffle of the words

$$ \begin{align*} ( \overline{k-s-1} )^{r_0} (s-1) ( \overline{k-s} )^{r_1} (s-2) ( \overline{k-s+1} )^{r_2} (s-3) \cdots 0 ( \overline{k-1} )^{r_s} \quad \text{and} \quad (k-1)^{n-r-s-1}, \end{align*} $$where $r_0 + r_1 + \cdots + r_s = r$. This means that $(c_2, \dots , c_n) \in {\mathcal {SS}}_{n-1,k,s}^{(r)}$. Conversely, for any $(c_2, \dots , c_n) \in {\mathcal {SS}}_{n-1,k,s}^{(r)}$, we see that $(a, c_2, \dots , c_n) \in {\mathcal {SS}}_{n,k,s}^{(r)}$ by prepending a $k-1$ to any $(n-1,k,s)$-staircase $(b_2, \dots , b_n) \geq (c_2, \dots , c_n)$.

The three bullet points above show that ${\mathcal {SS}}_{n,k,s}^{(r)}$ is a union of the claimed sets of words. The disjointness of this union follows since the first letters of the words in these sets are distinct.

We are ready to state the main result of this subsection: the codes of ordered set superpartitions are precisely the substaircase words. The key idea of the proof is to invert the map ${\mathrm {code}}: \sigma \mapsto (c_1, \dots , c_n)$, sending an ordered set partition to its coinversion code. The inverse map $\iota : (c_1, \dots , c_n) \mapsto \sigma $ is a variant on the insertion maps in the proof of Theorem 3.8.

Theorem 3.13. Let $n, k, s \geq 0$ be integers with $k \geq s$, and let $r \leq n-k$. The coinversion code map gives a well-defined bijection

Proof. Our first task is to show that the function ${\mathrm {code}}$ is well-defined, that is that ${\mathrm {code}}(\sigma ) \in {\mathcal {SS}}_{n,k,s}^{(r)}$ for any $\sigma \in {\mathcal {OSP}}_{n,k,s}^{(r)}$. To this end, let $\sigma = (B_1 \mid \cdots \mid B_k) \in {\mathcal {OSP}}_{n,k,s}^{(r)}$ with ${\mathrm {code}}(\sigma ) = (c_1, \dots , c_n)$. We associate a staircase $(b_1, \dots , b_n)$ to $\sigma $ as follows. Write the minimal elements $\min B_1, \dots , \min B_s$ in increasing order $i_1 < \cdots < i_s$, and set $b_{i_j} = s-j$ for each $j \in \{1,\ldots ,s\}$. If $1 \leq i \leq n$ and *i* is barred in $\sigma $, write $b_i = \overline {k-s-1+m_i}$, where $m_i = | \{ 1 \leq j \leq s \,:\, i_j < i \} |$. Finally, if $1 \leq i \leq n$, *i* is unbarred in $\sigma $, and *i* is not minimal in any of the first *s* blocks $B_1, \dots , B_s$, set $b_i = k-1$.

As an example of these concepts, consider $\sigma = (5, 7 \mid 1 \mid 3, \bar {4}, \bar {8} \mid \varnothing \mid \bar {2}, 6) \in {\mathcal {OSP}}_{8,5,3}^{(3)}$. The associated staircase is $(b_1, \dots , b_8) = (2, \bar {2}, 1, \bar {3}, 0, 4, 4, \bar {4})$. We have ${\mathrm {code}}(\sigma ) = (c_1, \dots , c_8) = (1, \bar {2}, 0, \bar {1}, 0, 4, 0, \bar {2})$, which has the same bar pattern as, and is componentwise $\leq $, the sequence $(b_1, \dots , b_8)$.

By construction, *b* is indeed a staircase. Furthermore, $b_i$ is barred if and only if $c_i$ is barred. It is also the case that $c_i \leq b_i$ for $1 \leq i \leq n$. One way to see this is to note that *b* is the unique set superpartition $\tau $ that has the same positive-height entries, the same zero-height entries and the same negative-height entries as $\sigma $ but with its code maximised in every coordinate. We can construct $\tau $ by writing the zero-height entries in increasing order from left to right and placing the other entries as far right as possible. In our example, we have $\tau = (1 \mid 3 \mid 5 \mid \varnothing \mid \bar {2}, \bar {4}, 6, 7, \bar {8}) \in {\mathcal {OSP}}_{8,5,3}^{(3)}$.

We leave it for the reader to verify that $(c_1, \dots , c_n)$ has the same bar pattern as $(b_1, \dots , b_n)$ and $(c_1, \dots , c_n) \leq (b_1, \dots , b_n)$ componentwise. This shows that the function ${\mathrm {code}}$ in the statement is well defined.

We show that ${\mathrm {code}}$ is a bijection by constructing its inverse map

as follows. The map $\iota $ starts with a sequence $(\varnothing \mid \cdots \mid \varnothing )$ of *k* copies of the empty set and builds up an ordered set superpartition by insertion.

Given a sequence $(B_1 \mid \cdots \mid B_k)$ of *k* possibly empty subsets of the alphabet $\{1, 2, \dots , \bar {1}, \bar {2}, \dots \}$, we assign the blocks $B_i$ the *unbarred labels* $0, 1, 2, \dots , k-1$ as follows. Moving from right to left, we assign the labels $0, 1, \dots , j-1$ to the *j* empty blocks among $B_1, \dots , B_s$. Then, moving from left to right, we assign the unlabeled blocks the labels $j, j+1, \dots , k-1$. An example of unbarred labels when $k = 7$ and $s = 4$ is as follows:

we draw unbarred labels below their blocks. The *barred* labels $\bar {0}, \bar {1}, \dots $, as assigned to the blocks $B_i$, where either $i> s$ or $B_i \neq \varnothing $ by moving left to right. In our example, the barred labels are

where the blocks $B_2 = B_3 = \varnothing $ do not receive a barred label because $2, 3 \leq s = 4$. Barred labels are written above their blocks.

Let $(c_1, \dots , c_n) \in {\mathcal {SS}}_{n,k,s}^{(r)}$. To define $\iota (c_1, \dots , c_n)$, we start with the sequence $(B_1 \mid \cdots \mid B_k) = (\varnothing \mid \cdots \mid \varnothing )$ of *k* empty blocks and iteratively insert *i* into the block with unbarred label $c_i$ (if $c_i$ is unbarred) or insert $\bar {i}$ into the block with barred label $c_i$ (if $c_i$