The asymptotics of the Touchard polynomials: a uniform approximation

Abstract

The asymptotic expansion of the Touchard polynomials $T_n(z)$ (also known as the exponential polynomials) for large $n$ and complex values of the variable $z$, where $|z|$ may be finite or allowed to be large like $O(n)$, has been recently considered in \cite{P1}. When $z=-x$ is negative, it is found that there is a coalesence of two contributory saddle points when $n/x=1/e$. Here we determine the expansion when $n$ and $x$ satisfy this condition and also a uniform two-term approximation involving the Airy function in the neighbourhood of this value. Numerical results are given to illustrate the accuracy of the asymptotic approximations obtained.