Explicit real algebraic functions which may have both compact and non-compact preimages

Abstract

As a pioneering work we construct explicit real algebraic functions which may have both compact and non-compact preimages. The author has obtained explicit real algebraic functions with preimages satisfying some nice conditions. More precisely, we have given answers to a considerably revised version of Sharko's question. Sharko originally asked whether we can have nice smooth functions whose Reeb graphs are as desired. The Reeb graph of a smooth function of a certain nice class is a natural graph whose underlying space is the space of all connected components. Such graphs can have some important topological information of the manifolds. Our answers are new in having real algebraic functions on non-compact manifolds with no boundaries. We also avoid using so-called existence theory and approximation theory whereas we also avoid in our previous studies.