I have a hard time proving the following proposition from John Lee's Introduction to Smooth Manifolds. I have found this question :If $b$ is a regular value of $f$, $f^{-1}(-\infty,b]$ is a regular domain? but I cannot understand the answer.
I think I need to show that $f^{-1}(-\infty, b]$ satiesfies the local $m$-slice condition for submanifolds with boundary, where $m= \dim M$.
Since $f^{-1}(-\infty,b)$, as an open subset of $M$, is an embedded submanifold, it must satisfy the local $m$-slice condition. The problem is $f^{-1}(b)$, which I predict satisfies the local half-slice condition. My attempt so far has been : since for all $p \in f^{-1}(b)$, $df_p$ is surjective, by Theorem 4.1 of the text, $p$ has a neighborhood $U$ such that $f|U$ is a smooth submersion. Then by the rank theorem, for $p$, there exist smooth charts $(W, \phi)$ for $U$ centered at $p$ and $(V, \psi)$ for $\mathbb{R}$ centered at $f(p)=b$ such that $f(W) \subset V$, in which $f$ has a coordinate representation of the form $f(x^1, \dots, x^m) = x^m$. How can I use this to show that $p$ is contained in the domain of a smooth chart $(A, \varphi =(x^i))$ such that $f^{-1}(b) \cap A$ is a $m$-dimensional half-slice, i.e. $\{(x^1, \dots, x^m) \in \varphi(A): x^m \ge 0\}$?
Best Answer
Maybe this is not relevant for you anymore but i'll post some of my work here.
Denote $R=f^{-1}((\infty,b]) \subseteq M$. We need to show that $R$ is a regular domain, which is by definition $R\subseteq M$ is a smooth manifold with boundary with the inclusion map $i : R \hookrightarrow M$ is a smooth embedding and also $i : R \hookrightarrow M$ is a proper map.
It is easy to show that if $i : R \hookrightarrow M$ is a topological embedding, then the topology of $R$ is the subspace topology. So let's equip $R$ with its subspace topology. Next we will find smooth (boundary) charts for $R=f^{-1}(-\infty,b) \cup f^{-1}(\{b\})$. Let $B = f^{-1}(\{b\})$.
So $R$ is a smooth $n$-manifold with boundary, and it is not hard to see that $i : R \hookrightarrow M$ is proper smooth embedding.