Level curves of open polynomial functions on the real plane.

Abstract

Let f : R(2) --> R be an open polynomial function. Then, f changes sign across V(f) (alternatively around a singular point of V(f)) and the function c : R --> N expressing the number c(lambda) of connected components of the lambda-level curve of f is lower semicontinuous; it has removable singularity at every value lambda which is critical and is not a real critical value at infinity for f.