On the number of real zeros of real entire functions with a non-decreasing sequence of the second quotients of Taylor coefficients

Abstract

For an entire function $f(z) = \sum_{k=0}^\infty a_k z^k,$ $a_k >0,$ we define the sequence of the second quotients of Taylor coefficients $Q := \left( \frac{a_k^2}{a_{k-1}a_{k+1}} \right)_{k=1}^\infty$. We find new necessary conditions for a function with a non-decreasing sequence $Q$ to belong to the Laguerre--P\'olya class of type I. We also estimate the possible number of nonreal zeros for a function with a non-decreasing sequence $Q.$