Chapoton's H-triangle identity for noncrystallographic root systems

Let Φ\Phi be a noncrystallographic root system of rank nn. Define the antichain generating function

H(Φ;s,t)=AA(Φ)sAΔtA.H(\Phi;s,t)=\sum_{A\in\mathcal{A}(\Phi)}s^{|A\cap\Delta|}t^{|A|}.

Let N ⁣C(Φ)N\!C(\Phi) be the noncrossing partition lattice and let

M(Φ;x,y)=σ,πNC(Φ)σπμ(σ,π)xnrk(σ)ynrk(π)M(\Phi;x,y)=\sum_{\substack{\sigma,\pi\in N\!C(\Phi)\sigma\leq\pi}}\mu(\sigma,\pi)x^{n-\operatorname{rk}(\sigma)}y^{n-\operatorname{rk}(\pi)}

be its Möbius generating function. Chapoton's H-triangle identity. Chapoton's HH-triangle is related to the MM-triangle by

H(Φ;s,t)=(1+(s1)t)nM(Φ;ss1,(s1)t1+(s1)t).H(\Phi;s,t)=(1+(s-1)t)^nM\left(\Phi;\frac{s}{s-1},\frac{(s-1)t}{1+(s-1)t}\right).

This conjecture asserts a numerical connection between antichains in the root poset and the Möbius function of the noncrossing partition lattice; the source states that it remains open.

Sources & referencesView supporting material

Primary source

Michael Cuntz and Christian Stump, “On root posets for noncrystallographic root systems”, arXiv:1212.2876 (2012).

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