Saddle-point integration of $C_\infty$ "bump" functions

Abstract

This technical note describes the application of saddle-point integration to the asymptotic Fourier analysis of the well-known $C_\infty$ "bump" function $\exp[-(1-x^2)^{-1}]$, deriving both the asymptotic decay rate $k^{-3/4} \exp(-\sqrt k)$ of the Fourier transform $F(k)$ and the exact coefficient. The result is checked against brute-force numerical integration and is extended to generalizations of this bump function.