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We apply the method of orbit harmonics to the set of break divisors and orientable divisors on graphs to obtain the central and external zonotopal algebras, respectively. We then relate a construction of Efimov in the context of cohomological Hall algebras to the central zonotopal algebra of a graph $G_{Q,\gamma }$ constructed from a symmetric quiver $Q$ with enough loops and a dimension vector $\gamma $. This provides a concrete combinatorial perspective on the former work, allowing us to identify the quantum Donaldson–Thomas (DT) invariants as the Hilbert series of the space of $S_{\gamma }$-invariants of the Postnikov–Shapiro slim subgraph space attached to $G_{Q,\gamma }$. The connection with orbit harmonics in turn allows us to give a manifestly nonnegative combinatorial interpretation to numerical DT invariants as the number of $S_{\gamma }$-orbits under the permutation action on the set of break divisors on $G$. We conclude with several representation-theoretic consequences, whose combinatorial ramifications may be of independent interest.

 

Let $V_1, V_2, V_3, \dots $ be a sequence of $\mathbb {Q}$-vector spaces where $V_n$ carries an action of $\mathfrak{S}_n$. Representation stability and multiplicity stability are two related notions of when the sequence $V_n$ has a limit. An important source of stability phenomena arises when $V_n$ is the $d^{th}$ homology group (for fixed $d$) of the configuration space of $n$ distinct points in some fixed topological space $X$. We replace these configuration spaces with moduli spaces of tuples $(W_1, \dots, W_n)$ of subspaces of a fixed complex vector space $\mathbb {C}^N$ such that $W_1 + \cdots + W_n = \mathbb {C}^N$. These include the varieties of spanning line configurations which are tied to the Delta Conjecture of symmetric function theory. 

Let $\Theta _n = (\theta _1, \dots , \theta _n)$ and $\Xi _n = (\xi _1, \dots , \xi _n)$ be two lists of $n$ variables, and consider the diagonal action of ${{\mathfrak {S}}}_n$ on the exterior algebra $\wedge \{ \Theta _n, \Xi _n \}$ generated by these variables. Jongwon Kim and the 2nd author defined and studied the fermionic diagonal coinvariant ring$FDR_n$ obtained from $\wedge \{ \Theta _n, \Xi _n \}$ by modding out by the ideal generated by the ${{\mathfrak {S}}}_n$-invariants with vanishing constant term. On the other hand, the 2nd author described an action of ${{\mathfrak {S}}}_n$ on the vector space with basis given by noncrossing set partitions of $\{1,\dots ,n\}$ using a novel family of skein relations that resolve crossings in set partitions. We give an isomorphism between a natural Catalan-dimensional submodule of $FDR_n$ and the skein representation. To do this, we show that set partition skein relations arise naturally in the context of exterior algebras. Our approach yields an ${{\mathfrak {S}}}_n$-equivariant way to resolve crossings in set partitions. We use fermions to clarify, sharpen, and extend the theory of set partition crossing resolution.

 

Abstract The superspace ring $\Omega _n$ is a rank n polynomial ring tensored with a rank n exterior algebra. Using an extension of the Vandermonde determinant to $\Omega _n$ , the authors previously defined a family of doubly graded quotients ${\mathbb {W}}_{n,k}$ of $\Omega _n$ , which carry an action of the symmetric group ${\mathfrak {S}}_n$ and satisfy a bigraded version of Poincaré Duality. In this paper, we examine the duality modules ${\mathbb {W}}_{n,k}$ in greater detail. We describe a monomial basis of ${\mathbb {W}}_{n,k}$ and give combinatorial formulas for its bigraded Hilbert and Frobenius series. These formulas involve new combinatorial objects called ordered set superpartitions . These are ordered set partitions $(B_1 \mid \cdots \mid B_k)$ of $\{1,\dots ,n\}$ in which the nonminimal elements of any block $B_i$ may be barred or unbarred.