Stapledon's positive-coefficient conjecture for equivariant H*-polynomials

Let PP be a GG-invariant polytope, and suppose that H(P;G)[t]H^*(P;G)[t] is an effective polynomial. Stapledon's positive-coefficient conjecture. If the coefficient of tmt^m in the ordinary hh^*-polynomial hP(t)h^*_P(t) is positive, then the coefficient of tmt^m in H(P;G)[t]H^*(P;G)[t] contains the trivial representation with non-zero multiplicity. The conjecture is refuted: for n4n\geq4, the hypersimplex Δ2,n\Delta_{2,n} supplies a counterexample, since its relevant ordinary coefficient is positive while the corresponding equivariant coefficient has zero trivial-representation multiplicity.

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Primary source

Oliver Clarke and Max Kölbl, “Equivariant Ehrhart Theory of Hypersimplices”, arXiv:2412.06524 (2025).

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