The numerical range of limits of real rational functions

Given two nonzero polynomials $f, g \in \mathbb{R}[x_1,\dots,x_n]$ vanishing at the origin $0 \in \mathbb{R}^n$, we study the numerical range of the limit $$ \lim_{x \to 0} \frac{f(x)}{g(x)}, $$ denoted by $\mathcal{R}_{f,g}$, which is defined as the set of all possible limits of the rational function $\displaystyle\frac{f}{g}$ as $x$ tends to $0$. When $n = 2$ and $g$ has a non-isolated zero at $0$, we describe all possible forms of $\mathcal{R}_{f,g}$ and provide an effective algorithm to compute this set based on Puiseux expansions. This is a follow-up to our recent results~\cite{Dinh2025}, in which we only considered the case when $0$ is an isolated zero of $g$.

Authors

Publication Details

Journal
arXiv (Cornell University)
Published
2026-10-27
DOI
https://doi.org/10.48550/arxiv.2610.36842
Primary Topic
Polynomial and algebraic computation
Type
preprint
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preprint

The numerical range of limits of real rational functions

Feng Guo, Sĩ Tiệp Đinh, Tien Son Pham, Hong Duc Nguyen
arXiv (Cornell University)
Polynomial and algebraic computation
preprint

The numerical range of limits of real rational functions

Feng Guo, Sĩ Tiệp Đinh, Tien Son Pham, Hong Duc Nguyen
preprint en

Abstract

Given two nonzero polynomials $f, g \in \mathbb{R}[x_1,\dots,x_n]$ vanishing at the origin $0 \in \mathbb{R}^n$, we study the numerical range of the limit $$ \lim_{x \to 0} \frac{f(x)}{g(x)}, $$ denoted by $\mathcal{R}_{f,g}$, which is defined as the set of all possible limits of the rational function $\displaystyle\frac{f}{g}$ as $x$ tends to $0$. When $n = 2$ and $g$ has a non-isolated zero at $0$, we describe all possible forms of $\mathcal{R}_{f,g}$ and provide an effective algorithm to compute this set based on Puiseux expansions. This is a follow-up to our recent results~\cite{Dinh2025}, in which we only considered the case when $0$ is an isolated zero of $g$.

arXiv (Cornell University)
Polynomial and algebraic computation
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