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We prove that cluster monomials of type $A_n$, when expanded in fan coordinates, exhibit log-concavity and unimodality. This is achieved by deriving a coefficient formula for laminar interval products and employing rank-dependent techniques: convolution for odd rank and reduction to integer lines for even rank. These findings provide partial affirmative answers to open conjectures regarding the log-concavity and unimodality of cluster monomials.
Cluster monomials in type $A_n$ fan coordinates are provably log-concave and unimodal, settling key conjectures.
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 $A_n$.
