Asymptotics of alternating harmonic series with attenuation

Abstract

We find the asymptotics of the series $\sum_{n=1}^\infty (-1)^n n^{-1} \exp(-t/n)$ as $t\to+\infty$. The answer is an oscillating function of $t$ dominated by $\exp(-(2\pi t)^{1/2})$. The intermediate step is to find the asymptotics of the two-dimensional Fourier transform $\hat F(\xi)$ of the function $F(x)=(1+\exp(\|x\|^2))^{-1}$ as $\|\xi\|\to\infty$.