On the clustering of Padé zeros and poles of random power series

Abstract

We estimate non-asymptotically the probability of uniform clustering around the unit circle of the zeros of the $[m,n]$-Padé approximant of a random power series $f(z)={\sum }_{j=0}^{\infty }{a}_j{z}^j$ for $a_j$ independent, with finite first moment, and Lévy function satisfying $L(a_j,\epsilon )\le K\epsilon$. Under the same assumptions we show that almost surely $f$ has infinitely many zeros in the unit disc, with the unit circle serving as a natural boundary for $f$. For $R_m$ the radius of the largest disc containing at most $m$ zeros of $f$, a deterministic result of Edrei implies that in our setting the poles of the $[m,n]$-Padé approximant almost surely cluster uniformly at the circle of radius $R_m$ as $n\to \infty$ and $m$ stays fixed, and we provide almost sure rates of converge of these $R_m$'s to $1$. We also show that our results on the clustering of the zeros hold for log-concave vectors $(a_j)$ with not necessarily independent coordinates.