Equivalence of Milnor and Milnor-L\^e fibrations for real analytic maps

Abstract

In [22] Milnor proved that a real analytic map $f\colon (R^n,0) \to (R^p,0)$, where $n \geq p$, with an isolated critical point at the origin has a fibration on the tube $f|\colon B_\epsilon^n \cap f^{-1}(S_\delta^{p-1}) \to S_\delta^{p-1}$. Constructing a vector field such that, (1) it is transverse to the spheres, and (2) it is transverse to the tubes, he "inflates" the tube to the sphere, to get a fibration $\varphi\colon S_\epsilon^{n-1} \setminus f^{-1}(0) \to S^{p-1}$, but the projection is not necessarily given by $f/ \|f\|$ as in the complex case. In the case $f$ has isolated critical value, in [9] it was proved that if the fibres inside a small tube are transverse to the sphere $S_\epsilon$, then it has a fibration on the tube. Also in [9], the concept of $d$-regularity was defined, it turns out that $f$ is $d$-regular if and only if the map $f/|f\|\colon S_\epsilon^{n-1} \setminus f^{-1}(0) \to S^{p-1}$ is a fibre bundle equivalent to the one on the tube. In this article, we prove the corresponding facts in a more general setting: if a locally surjective map $f$ has a linear discriminant $\Delta$ and a fibration on the tube $f|\colon B_\epsilon^n \cap f^{-1}(S_\delta^{p-1} \setminus \Delta) \to S_\delta^{p-1} \setminus \Delta$, then $f$ is $d$-regular if and only if the map $f/ \|f\|\colon S_\epsilon^{n-1} \setminus f^{-1}(\Delta) \to S^{p-1} \setminus \mathcal{A}$ (with $\mathcal{A}$ the radial projection of $\Delta$ on $S^{p-1}$) is a fibre bundle equivalent to the one on the tube. We do this by constructing a vector field $\tilde{w}$ which inflates the tube to the sphere in a controlled way, it satisfies properties analogous to the vector field constructed by Milnor in the complex setting: besides satisfying (1) and (2) above, it also satisfies that $f/ \|f\|$ is constant on the integral curves of $\tilde{w}$.