On a positivity conjecture of Fasi and Higham for Padé approximants to the matrix exponential

Abstract

We study the positivity conjecture of Fasi and Higham [SIAM J. Matrix Anal. Appl. 40 (2019), Conjecture 3.2], which concerns the Taylor coefficients of the reciprocal of the denominator $q_{k,m}(z)$ of the Padé approximant to the matrix exponential. The conjecture asserts that these coefficients are positive whenever $m$ is odd. We prove full positivity for $k=0$ and eventual positivity whenever the unique positive real zero of $q_{k,m}$ is the unique zero of minimal modulus. We also give an explicit computable sufficient threshold beyond which all coefficients are positive. Using Hermite scaling with leading-coefficient normalization, we prove that this spectral minimality holds for each fixed odd $m$ and all sufficiently large $k$. The unrestricted conjecture is nevertheless false. We give exact rational counterexamples and prove a transient scaling theorem showing that on the scale $n\asymp\sqrt{k+1}$ the normalized coefficients converge to an explicit oscillatory Hermite profile. For $m=5$ this profile is negative at one point, yielding infinitely many counterexamples. The counterexamples are not diagonal, and the diagonal case $k=m$, which is the case relevant in practice, remains open. These results do not imply a failure of the Fasi--Higham algorithm. Rather, they delimit the range in which the auxiliary positivity conjecture can be used as a theorem.