Strict Total Positivity from Spectral Darboux and Toeplitz Smoothing Mechanisms

Abstract

We prove two strict total-positivity results by isolating two strictification mechanisms. The first is a spectral Darboux mechanism: an induction converts positivity and ordered endpoint asymptotics for a one-dimensional spectral family into positive Wronskians and hence into strict total positivity. As an application, the modified-Bessel kernel $K(x,s)=I_s(x)$, $x>0$, $s\ge 0$, is strictly totally positive of infinite order. This proves the real-order determinant positivity asked for by Buchstaber and Glutsyuk after their nonnegative-integer-order theorem. The second mechanism is discrete Toeplitz smoothing: every two-sided Polya-frequency sequence is a pointwise limit of totally positive Polya-frequency sequences. This gives a product-topology answer to Question 12.2 of Belton, Guillot, Khare, and Putinar. The density statement is in the product topology on $\mathbb{R}^{\mathbb{Z}}$; no uniform, weighted, or norm-density assertion is made.