The shifted generalized forest-matrix Hankel-total positivity conjecture

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Let F~(q,y,z)\tilde{\mathsf{F}}(q,y,z) be the generalized forest matrix, and define Fˉ(q,y,z)=F~(q,y,z)diag((qk(k1)/2)k0)\bar{\mathsf{F}}(q,y,z)=\tilde{\mathsf{F}}(q,y,z)\operatorname{diag}((q^{k(k-1)/2})_{k\geq0}). Let Fˉn(x;q,y,z)=k=0nfˉn,k(q,y,z)xk\bar{\mathsf{F}}_n(x;q,y,z)=\sum_{k=0}^n\bar{f}_{n,k}(q,y,z)x^k, with Fˉ0(x;q,y,z)=1\bar{\mathsf{F}}_0(x;q,y,z)=1. The shifted generalized forest-matrix conjecture. The Hankel matrix

H(Fˉn+n(x;q,y,z))n,n0H_{\infty}(\bar{\mathsf{F}}_{n+n'}(x;q,y,z))_{n,n'\geq0}

is coefficientwise totally positive jointly in q,x,y,zq,x,y,z. The unshifted generalized sequence fails coefficientwise total positivity, whereas computations up to 5×55\times5 suggest the diagonal modification restores it; the conjecture remains open.

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Sources & referencesView supporting material

Primary source

Tomack Gilmore, “Trees, forests, and total positivity: I. q-trees and q-forests matrices”, arXiv:2106.00656 (2021).

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