Shifted Hankel total positivity for bivariate forest polynomials

Let fn,k(w)f^\sharp_{n,k}(w) and Fn(x,w)=k=0nfn,k(w)xkF_n^\sharp(x,w)=\sum_{k=0}^n f^\sharp_{n,k}(w)x^k be as above. A polynomial sequence is coefficientwise Hankel-totally positive when all minors of its Hankel matrix have coefficientwise nonnegative coefficients. Shifted bivariate-forest Hankel-total-positivity conjecture. (b) The sequence F=(Fn(x,1+w))n0{\bm{F}}^\sharp=\bigl(F_n^\sharp(x,-1+w')\bigr)_{n\geq 0} is coefficientwise Hankel-totally positive in x,wx,w'. (c) The sequence F=(fn+1,1(1+w))n0{\bm{F}}^{\sharp\triangle}=\bigl(f^\sharp_{n+1,1}(-1+w')\bigr)_{n\geq 0} is coefficientwise Hankel-totally positive in ww'. These shifted claims are proposed after the corresponding matrix claim fails at order two; the source gives no resolution.

Sources & referencesView supporting material

Primary source

Alan D. Sokal, “Total positivity of some polynomial matrices that enumerate labeled trees and forests, I. Forests of rooted labeled trees”, arXiv:2105.05583 (2022).

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