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arXiv · 2607.22301

Positivity of stretched Littlewood-Richardson coefficients for partitions of length at most five

Alper Ferudun
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Abstract

For partitions lambda, mu, nu the Littlewood-Richardson coefficient stretches to a function P(t) = c(t*nu; t*lambda, t*mu) which, by a theorem of Derksen and Weyman, is a polynomial in t. King, Tollu and Toumazet conjectured that P has no negative coefficient. The conjecture is known only for c <= 2 and is otherwise open. We prove it for all triples whose parts number at most five. For at most four parts the proof is structural: a period-one dilation transfers the Berline-Vergne local Ehrhart formula from an integral dilate back to the rational hive polytope, and every two-dimensional transverse cone spanned by rank-four rhombus normals has positive weight (minimum 1/9). Five parts need two further ingredients. A closed-chamber transfer lemma, resting on Rassart's polynomiality of the coefficients on the closed cones of a chamber complex, reduces positivity for a fixed number of parts to full-dimensional hives. For full-dimensional five-part hives all coefficients but the linear one were already known to be positive; the linear one is where pointwise positivity of the local weights fails -- an edge cone of weight -347/109824 occurs on an actual hive -- and its nonnegativity is proved by an exact global certificate: a rational correction supported on 6,903 two-face types, balanced by the Minkowski closure of each two-face polygon, which makes every one of the 87,127 closed edge cones adjacent to them nonnegative. The same local method yields a rank-uniform consequence: for every full-dimensional hive polytope of any rank the top four Ehrhart coefficients are positive. We close by recording the exact general frontier and the obstructions that rule out the standard shortcuts. All finite verifications are exact and accompanied by independent replay scripts.

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Alper Ferudun. 2026-07-24. Positivity of stretched Littlewood-Richardson coefficients for partitions of length at most five. https://arxiv.org/abs/2607.22301

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