Continuous Narayana Interpolations: Hankel Total Positivity and Log-Convexity Preservation

Source abstract

We study a continuous interpolation between the type $B$ and shifted type $A$ Narayana triangles and its extension through affine moments of the continuous $q$-ultraspherical (Rogers) measure. For every fixed $0<q<1$ and $-1<β<1$, these affine moments form a coefficientwise Hankel-totally positive sequence in the polynomial variable $t$. The proof rests on a general continuant criterion: decreasing nonnegative off-diagonal weights and coefficientwise positivity of the leading continuants imply total positivity of the full tridiagonal matrix. A telescoping identity gives positive certificates for shifted continuants and stability under coordinatewise decreases of the weights. We determine the exact power of $t$, palindromicity, and extreme coefficients of arbitrary Hankel minors. Classical limits recover the continuous Narayana family, including its moment representation and gamma positivity. We further prove that the continuous Narayana transformations preserve strong $t$-log-convexity for every real $θ\ge0$, and that the $d$-Hoggatt transformations have the same preservation property for every integer $d\ge1$. The preservation proofs use the Zhu--Sun criterion and a mixed-minor one-sign-change argument whose local ingredient is a hypergeometric hitting-probability inequality.