Pinnacle-set refinement of gamma-positivity for inverse descent polynomials

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For n≥1n\geq1 and S⊆[n]S\subseteq[n], let Pin⁡(π)\operatorname{Pin}(\pi) be the set of values at peak positions of π∈Sn\pi\in\mathfrak{S}_n, and define

A^n,Sides⁡(t)=∑π∈SnPin⁡(π)=Stides⁡(π)+1.\hat{A}_{n,S}^{\operatorname{ides}}(t)=\sum_{\substack{\pi\in\mathfrak{S}_n\\\\ \operatorname{Pin}(\pi)=S}}t^{\operatorname{ides}(\pi)+1}.

A polynomial is γ\gamma-positive with center of symmetry (n+1)/2(n+1)/2 if its gamma-basis coefficients relative to that center are nonnegative. Pinnacle-set gamma-positivity conjecture. For all n≥1n\geq1 and S⊆[n]S\subseteq[n], the polynomial A^n,Sides⁡(t)\hat{A}_{n,S}^{\operatorname{ides}}(t) is γ\gamma-positive with center of symmetry (n+1)/2(n+1)/2. This is a refinement of the peak-number version by fixing the entire pinnacle set.

References

Primary source

Ira M. Gessel and Yan Zhuang, “Two-sided permutation statistics via symmetric functions”, arXiv:2306.15785 (2024).

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