Asymptotics of Multivariate Sequences II: Multiple Points of the Singular Variety

Source abstract

Let $F(\b{z})=\sum_\b{r} a_\b{r}\b{z^r}$ be a multivariate generating function that is meromorphic in some neighbourhood of the origin of $\mathbb{C}^d$ , and let $\sing$ be its set of singularities. Effective asymptotic expansions for the coefficients can be obtained by complex contour integration near points of $\sing$ . In the first article in this series, we treated the case of smooth points of $\sing$ . In this article we deal with multiple points of $\sing$ . Our results show that the central limit (Ornstein–Zernike) behaviour typical of the smooth case does not hold in the multiple point case. For example, when $\sing$ has a multiple point singularity at $(1, \ldots, 1)$ , rather than $a_\b{r}$ decaying as $|\b{r}|^{-1/2}$ as $|\b{r}| \to \infty$ , $a_\b{r}$ is very nearly polynomial in a cone of directions.