Armstrong's F=M conjecture for generalised non-crossing partitions

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Let Φ\Phi be a finite root system of rank nn, let mm be a positive integer, and let FΦm(x,y)F^m_\Phi(x,y) be the FF-triangle of the generalised cluster complex. Let NCm(Φ)NC^m(\Phi) be the associated generalised non-crossing partition poset, with M-triangle

MΦm(x,y)=∑u,w∈NCm(Φ)μ(u,w)xrk⁡uyrk⁡w.M^m_\Phi(x,y)=\sum_{u,w\in NC^m(\Phi)}\mu(u,w)x^{\operatorname{rk}u}y^{\operatorname{rk}w}.

Write (NCm(Φ))∗(NC^m(\Phi))^* for its dual poset, with rank function rk⁡∗\operatorname{rk}^* and Möbius function μ∗\mu^*. Armstrong's F=MF=M conjecture. For every finite root system Φ\Phi of rank nn,

FΦm(x,y)=ynMΦm(1+yy−x,y−xy).F^m_\Phi(x,y)=y^nM^m_\Phi\left(\frac{1+y}{y-x},\frac{y-x}{y}\right).

Equivalently,

(1−xy)nFΦm(x(1+y)1−xy,xy1−xy)=∑u,w∈(NCm(Φ))∗μ∗(u,w)(−x)rk⁡∗w(−y)rk⁡∗u.(1-xy)^nF^m_\Phi\left(\frac{x(1+y)}{1-xy},\frac{xy}{1-xy}\right)=\sum_{u,w\in(NC^m(\Phi))^*}\mu^*(u,w)(-x)^{\operatorname{rk}^*w}(-y)^{\operatorname{rk}^*u}.

This conjecture relates the refined face enumeration of generalised cluster complexes to Möbius-function enumeration in generalised non-crossing partition posets. The displayed identity was proved in the paper for type DnD_n, but the assertion for arbitrary finite root systems is not established here.

References

Primary source

Christian Krattenthaler and Thomas Müller, “Decomposition numbers for finite Coxeter groups and generalised non-crossing partitions”, arXiv:0704.0199 (2009).

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