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arXiv · 2610.09312

Log-concavity and unimodality of cluster monomials in fan coordinates of type $A_n$

Zhichao Chen·Zhe Sun
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Abstract

We prove that every cluster monomial of type $A_n$ is log-concave and unimodal with respect to its Laurent expansion in a fan initial cluster. We derive a coefficient formula for laminar interval products arising from compatible diagonals in the polygon model. In even rank, we reduce log-concavity in fan coordinates to log-concavity along integer lines. In odd rank, we use convolution to handle the coefficient sums arising from the fan substitution. This gives partial affirmative answers to the log-concavity conjecture of Chen-Huang-Sun and the unimodality conjecture of Chen. Our result also complements Shen's study on the supports of products of canonical basis elements for cluster X-varieties of type $A_n$.

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Zhichao Chen, Zhe Sun. 2026-10-07. Log-concavity and unimodality of cluster monomials in fan coordinates of type $A_n$. https://arxiv.org/abs/2610.09312

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