Upper Bounds for the Renewal Function Via Fourier Methods

Abstract

Stone has used Fourier analytic methods to show that the renewal function $U(x) = \sum^\infty_0 F^{n\ast}(x)$ for a random variable $X$ with distribution function $F$, finite second moment and positive mean $\lambda^{-1} = EX$, is bounded above by $\lambda x_+ + C\lambda^2EX^2$ for a universal constant $C, 1 \leqq C < 3$. This paper refines his method to prove that $C < 2.081$, and shows that within certain constraints the smallest upper bound on $C$ that the method will yield is 1.809. Various authors' work on the simpler case where $X \geqq 0$ is summarized: the best result is the earliest published one, due to Lorden, who showed that then $C = 1$.