Dual Shellability of Admissible Sets

• The paper establishes that dual shellability, using explicit dual EL-labeling, ensures Cohen–Macaulayness and resolves longstanding conjectures in arithmetic geometry.
• It employs a methodology that constructs lexicographically minimal chains in posets, unifying combinatorial, topological, and algebraic approaches.
• The paper demonstrates significant implications for admissible sets, providing uniform labeling strategies that enhance the understanding of topological invariants and ideal-theoretic properties.

Dual shellability of admissible sets is a combinatorial-topological property that arises at the interface of simplicial complexes, poset theory, and the algebraic geometry of moduli spaces. It refers to the existence of a lexicographic shelling (usually a dual EL-labeling or an equivalent combinatorial structure) on posets and complexes whose elements are “admissible” with respect to certain group-theoretic, lattice-theoretic, or matroidal criteria. Dual shellability plays a decisive role in guaranteeing strong algebraic and topological properties, notably the Cohen–Macaulayness of geometric models in arithmetic geometry, as well as favorable homological and ideal-theoretic features in commutative algebra. Recent work has achieved a unified understanding and explicit proofs of dual shellability for broad classes of admissible sets, resolving long-standing conjectures and providing explicit labeling constructions.

1. Definitions and General Framework

Dual shellability generalizes the classical notion of shellability, which involves ordering the maximal faces (facets) of a complex to allow a sequential, combinatorially governed construction that preserves topological and homological regularity (such as Cohen–Macaulayness). In the dual context, the shelling order is replaced by a dual EL-labeling: an edge-labeling η on the Hasse diagram of a bounded graded poset P (often the augmented admissible set of interest) such that in every interval [w, w′] ⊂ P there exists a unique (downward) maximal chain which is label-increasing and lexicographically minimal among all maximal chains in that interval.

In the context of Iwahori–Weyl groups, for a dominant coweight μ, the admissible set

$$\operatorname{Adm}(\mu) = \{ w \in \widetilde{W} : w \leq t^{\mu'} \text{ for some } \mu' \sim \mu \}$$

is augmented to $$\widehat{\operatorname{Adm}(\mu)} = \operatorname{Adm}(\mu) \sqcup \{\hat{1}\}$$, and the dual shellability property is established by constructing an explicit dual EL-labeling on this poset (He et al., 15 Sep 2025).

More broadly, "admissible sets" may arise as independent sets in matroids, simplicial complexes fulfilling flag or partitionability properties, or as certain subposets encoding stratifications of moduli spaces.

2. Obstructions, Hereditary Properties, and Duality

A central insight is the classification of “obstructions” to shellability, which are minimal complexes (with respect to induced subcomplexes) that are not shellable but whose restrictions are. For simplicial complexes of dimension ≤2 and in the class of flag complexes, the sets of obstructions to shellability, partitionability, and sequential Cohen–Macaulayness coincide (Hachimori et al., 2010). In these cases, hereditary-shellability, hereditary-partitionability, and hereditary-sequential Cohen–Macaulayness are equivalent properties. This equivalence is preserved under dualization for complexes equipped with a natural combinatorial duality, such as independence complexes or clique complexes formed from graphs.

Passing to dual admissible sets or dual complexes does not introduce new obstructions; the “minimal bad examples” remain the same. Thus, dual shellability is governed by the same combinatorial data as shellability in the primal context, providing a robust invariance under duality operations in various classes of combinatorial structures.

3. Dual EL-Labelings and Explicit Constructions

Recent breakthroughs have provided explicit labeling strategies to prove dual shellability. For the admissible set $\operatorname{Adm}(\mu)$ in Iwahori–Weyl groups, the dual EL-labeling is constructed in two steps. Edges within $\operatorname{Adm}(\mu)$ are labeled using a total order on positive affine roots, while edges from $\hat{1}$ to top elements receive distinct designated labels (η_a). The strict order $$(\text{negative root},k') \prec \eta_a \prec (\text{positive root},k)$$ is enforced to separate layers in the poset. This labeling sequentially builds the augmented admissible set and ensures that each maximal chain is uniquely determined by its lex-minimal, increasing label sequence (He et al., 15 Sep 2025). The same strategy, or its analogues, has been applied in matroidal lattices, posets from stratifications of symmetric varieties, and independence complexes via order complexes and Alexander duality (Woodroofe, 2011, Hersh et al., 2022).

The dual EL-labeling property is inherently compatible with link and restriction operations, which commute with dualization, thereby allowing the machinery to operate in both primal and dual settings. Notably, in the uncrossing posets of electrical network theory, the existence of a dual EC-labeling, which satisfies the unique-earliest property, guarantees dual CL-shellability (Hersh et al., 2022).

4. Algebraic and Geometric Implications

Dual shellability is a powerful combinatorial certificate for the Cohen–Macaulayness of associated schemes or Stanley–Reisner rings. When a stratifying poset (such as the augmented admissible set for local models of Shimura varieties) is dual shellable, the process of sequentially adding strata—guided by the explicit labeling—preserves Cohen–Macaulayness at each stage. This was recently used to resolve Görtz's conjecture, establishing that the special fibers of local models with Iwahori level structure are Cohen–Macaulay for all reductive groups, including those with residue characteristic 2 and non-reduced root systems (He et al., 15 Sep 2025).

This combinatorial approach bypasses case-by-case geometric methods (Frobenius splittings, compactifications), giving a uniform, elementary proof and opening new routes for applications in the geometry of Shimura varieties and moduli spaces of shtukas.

5. Connections to Broader Shellability Frameworks

Dual shellability is situated in a broader ecosystem of shellability-type properties. Many recent works have developed equivalence hierarchies among schemes for proving (dual) shellability: from classic CL-shellability based on recursive atom orderings (RAOs), to more general CC-shellability and topological chain labeling approaches (TCL-shellability) (Hersh et al., 2022, Lacina et al., 28 Mar 2025). Recursive first atom sets (RFAS) offer a minimal recursive data structure that is necessary for CC-shellability and sufficient for shellability (Lacina et al., 28 Mar 2025).

In poset and semimodular lattice frameworks, dual shellability may be realized in both the primal and dual orderings, with results transferring between the order complex and the face lattice of the dual (Pratihar et al., 11 Jul 2024, Woodroofe, 2011). Matroid theory on power lattices also demonstrates a dual basis exchange property, underpinning dual shellability for admissible sets seen as independent sets within these structures (Pratihar et al., 11 Jul 2024).

6. Applications and Further Ramifications

Applications of dual shellability extend through several domains:

• Arithmetic geometry: Establishes Cohen–Macaulayness of local models and the singularities of Shimura varieties (He et al., 15 Sep 2025).
• Algebraic combinatorics: Via dual shellability and Alexander duality, results such as the linear resolution (i.e., linear quotients) property for certain classes of monomial ideals (edge ideals of hypergraphs describing higher independence complexes) and sequential Cohen–Macaulayness for Stanley–Reisner rings are ensured (Ghosh et al., 10 May 2025, Pratihar et al., 11 Jul 2024).
• Combinatorial topology: Provides complete sets of obstructions for hereditary shellability, partitionability, and Cohen–Macaulayness, especially in flag complexes and low dimensions (Hachimori et al., 2010).
• Poset theory: Underlies the recursive construction of shellings and dual shellings in bounded posets, with particular attention to the admissible sets arising in combinatorial models of electrical networks and symmetric varieties (Hersh et al., 2022, Bingham et al., 2023).
• Potential extensions: The explicit labeling and construction methods enable further generalizations, including conjectured dual EL-shellability of subsets of admissible sets related to basic loci in Shimura varieties, and provide a foundation for analogous results in moduli of shtukas and similar spaces (He et al., 15 Sep 2025).

A plausible implication is that, due to the explicit nature of the labeling procedures and the transferability of the key combinatorial structures under dualization, any admissible set arising from a Coxeter-type group or a combinatorial model with well-behaved dual posets is likely to admit a dual shelling, implying robust topological and algebraic regularity for associated geometric and algebraic objects.

7. Summary Table: Core Properties and Their Duals

Property	Dual Statement / Consequence	Source
Shellability	Dual shellability (dual EL-labeling)	(He et al., 15 Sep 2025, Hersh et al., 2022)
Sequential Cohen–Macaulayness	Sequential Cohen–Macaulayness of Alexander dual	(Hachimori et al., 2010, Pratihar et al., 11 Jul 2024)
Cohen–Macaulayness (Geometric)	Cohen–Macaulayness via combinatorial dual shelling	(He et al., 15 Sep 2025)
Obstruction sets (dimension ≤2)	Dual obstructions coincide	(Hachimori et al., 2010)
Edge ideal linear quotients	Linear resolution via Alexander dual shellability	(Ghosh et al., 10 May 2025)

This framework demonstrates that dual shellability has become a fundamental connector from the combinatorics of admissible sets to their topological, geometric, and algebraic properties, with explicit algorithms and labeling constructions now available to verify this property in a wide array of contexts.