Sokal's reversed second-order Eulerian polynomial conjecture

For r1r\geq1, define the rrth-order Eulerian polynomials by E0[r](x)=1E_0^{[r]}(x)=1 and, for n1n\geq1,

En[r](x)=xAn(r)(x)=k=0n1langlenkrangle(r)xk+1.E_n^{[r]}(x)=xA_n^{(r)}(x)=\sum_{k=0}^{n-1}\genfrac{\langle}{\rangle}{0pt}{}{n}{k}^{(r)}x^{k+1}.

Define the reversed polynomial by

En[r](x)=xnEn[r](1/x).E_n^{[r]*}(x)=x^nE_n^{[r]}(1/x).

Sokal's reversed second-order Eulerian conjecture. The sequence (En[2](x))n0(E_n^{[2]*}(x))_{n\geq0} is coefficientwise Hankel-totally positive in xx.

The ordinary rrth-order Eulerian polynomials already have coefficientwise Hankel-total positivity via a branched Stieltjes-type continued fraction. The assertion for the reversed second-order sequence is presented as a separate conjecture, with no resolution given in the source.

Sources & referencesView supporting material

Primary source

Bao-Xuan Zhu, “Coefficientwise Hankel-total positivity of the row-generating polynomials for the output matrices of certain production matrices”, arXiv:2202.03793 (2024).

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