Parabolic aligned elements form a lattice quotient of parabolic weak order

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Let (W,S)(W,S) be a finite Coxeter system, let J⊆SJ\subseteq S, and let c∈Wc\in W be a Coxeter element. An element of WJW^J is (WJ,c)(W^J,c)-aligned if, whenever tα<taα+bβ<tβt_{\alpha}<t_{a\alpha+b\beta}<t_{\beta} in Inv⁡(w∘J(c))\operatorname{Inv}(\mathbf{w_{\circ}^{J}(c)}), where α,β∈Φ+\alpha,\beta\in\Phi^{+} and a,ba,b are positive integers, the condition taα+bβ∈Cov⁡(w)t_{a\alpha+b\beta}\in\operatorname{Cov}(w) implies tα∈Inv⁡(w)t_{\alpha}\in\operatorname{Inv}(w). Write Align⁡(WJ,c)\operatorname{Align}(W^J,c) for the set of such elements, and let Weak⁡(Align⁡(WJ,c))\operatorname{Weak}(\operatorname{Align}(W^J,c)) denote the weak-order poset on this set. Parabolic lattice conjecture. For any finite Coxeter system (W,S)(W,S), any J⊆SJ\subseteq S, and any Coxeter element c∈Wc\in W, the poset

Weak⁡(Align⁡(WJ,c))\operatorname{Weak}\bigl(\operatorname{Align}(W^{J},c)\bigr)

is a lattice. Moreover, it is a lattice quotient of Weak⁡(WJ)\operatorname{Weak}(W^{J}). This would extend the known parabolic Tamari-lattice result from the symmetric group to arbitrary finite Coxeter systems and parabolic quotients; the claim was motivated by computer experiments, but no resolution is supplied here.

References

Primary source

Henri Mühle and Nathan Williams, “Tamari Lattices for Parabolic Quotients of the Symmetric Group”, arXiv:1804.02761 (2019).

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