Motivic Milnor Fibres in Families of Real Singularities

Abstract

In this article we prove two results concerning the motivic Milnor fibres $S^{\epsilon}(f)$ associated to a map germ $f: (\mathbb{R}^n,0)\to(\mathbb{R},0)$, defined by G. Comte and G. Fichou. Firstly, we prove that if $f,g:(\mathbb{R}^n,0)\to(\mathbb{R},0)$ are arc-analytically equivalent germs of Nash functions then the virtual Poincar\'e polynomial of the corresponding motivic Milnor fibres are equal. This extends (and provides a new proof) of a result of G. Fichou. Secondly, let $T\subset\mathbb{R}^m$ be a real algebraic set and $f: T\times(\mathbb{R}^n,0)\to\mathbb{R}$ a polynomial function of polynomial map germs such that $f(t,0)=0$ for any $t\in T$. Then we prove that there exists a locally finite real analytic stratification $\mathcal{S}$ of $T$ such that if $S\in\mathcal{S}$ is a stratum then $f_t\sim_a f_{t'}$ are arc-analytically equivalent, for any $t,t'\in S$. Furthermore, if $T$ is compact then the stratification $\mathcal{S}$ can be taken to be finite. These two results imply in particular that the virtual Poincar\'e polynomials of the respective zeta functions are equal i.e $\beta(S^{\epsilon}(f_t))=\beta(S^{\epsilon}(f_{t'}))$.