The local $h$-vector of the cluster subdivision of a simplex

The cluster complex $Δ(Φ)$ is an abstract simplicial complex, introduced by Fomin and Zelevinsky for a finite root system $Φ$. The positive part of $Δ(Φ)$ naturally defines a simplicial subdivision of the simplex on the vertex set of simple roots of $Φ$. The local $h$-vector of this subdivision, in the sense of Stanley, is computed and the corresponding $γ$-vector is shown to be nonnegative. Combinatorial interpretations to the entries of the local $h$-vector and the corresponding $γ$-vector are provided for the classical root systems, in terms of noncrossing partitions of types $A$ and $B$. An analogous result is given for the barycentric subdivision of a simplex.

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