Coefficientwise Hankel-total positivity from a universal cumulant lifting

Abstract

Using probabilistic methods, we establish universal cumulant lifting criteria for Stieltjes and Hamburger moment sequences. In the Stieltjes case, we characterize coefficientwise Hankel total positivity after convolution with every initial moment sequence by requiring each once-shifted cumulant sequence to be Stieltjes. We also prove equivalence with the numerical moment property at every nonnegative intensity. In the Hamburger case, we use twice-shifted cumulants to obtain coefficientwise-positive leading Hankel determinants, a degree bound, an explicit highest-degree component, and drift invariance. We work with formal series and require neither exponential moments nor moment determinacy. We establish composition, binomial-convolution, and substitution results, and apply them to implicit equations and autonomous differential equations, tree and forest families, and lognormal compound-Poisson moments in a unified approach. From known moment sequences, we obtain general coefficientwise Hankel-totally positive families containing the classical Bessel, Lah, and Laguerre polynomials. We not only give new proofs of coefficientwise Hankel-total positivity of many combinatorial polynomials, but also prove coefficientwise Hankel total positivity for the component polynomials of functional digraphs confirming a conjecture of Sokal and establish fixed-shape Schläfli--Gessel--Seo polynomials positivity supporting another conjecture of Sokal.