Computing the non-properness set of real polynomial maps in the plane

Abstract

We introduce novel mathematical and computational tools to develop a complete algorithm for computing the set of non-properness of polynomials maps in the plane. In particular, this set, which we call \emph{the Jelonek set}, is a subset of ${\mathbb{K}}^2$ where a dominant polynomial map $f:{\mathbb{K}}^2\to {\mathbb{K}}^2$ is not proper; $\mathbb{K}$ could be either $\mathbb{C}$ or $\mathbb{R}$. Unlike all the previously known approaches we make no assumptions on $f$ whenever $\mathbb{K}=\mathbb{R}$; this is the first algorithm with this property. The algorithm takes into account the Newton polytopes of the polynomials. As a byproduct we provide a finer representation of the set of non-properness as a union of semi-algebraic curves, that correspond to edges of the Newton polytopes, which is of independent interest. Finally, we present a precise Boolean complexity analysis of the algorithm and a prototype implementation in Maple.

arXiv commentMajor revision made. To appear in Vietnam Journal of Mathematics. 32 pages, 5 figures, comments are welcome!