# Show that the fibre $\pi^{-1}(p)$ is a regular submanifold

Exercise 6.4 of the book Manifolds and Differential Geometry (Jeffrey M. Lee, 2009) states the following:

Show that if $$(E,M,\pi, F)$$ is a (smooth) fiber bundle, then $$\pi: E \rightarrow M$$ is a submersion and each fiber $$\pi^{-1}(p)$$ is a regular submanifold which is diffeomorphic to $$F$$. Show that if both $$F$$ and $$M$$ are connected, then E is connected.

I think I can proof the first statement: Because $$(E,M,\pi,F)$$ is a fibre bundle, for each $$p\in M$$ there exists a neighborhood $$U$$ which contains $$p$$ and a diffeomorphism $$\Psi:\pi^{-1}(U) \rightarrow U\times F$$ such that $$\pi|_U=\Psi\circ \mbox{proj}_U$$ (composition is by the right). Hence $$d\pi|_U = d\Psi\circ d\,\mbox{proj}_U$$; since $$\Psi$$ is a diffeomorphism, $$d\Psi$$ is bijective. Clearly $$d\,\mbox{proj}_U$$ has constant rank equal to $$\mbox{dim }(M)$$ and thus it follows that $$\pi$$ is a submersion.

In order to see that $$\pi^{-1}(p)$$ for $$p\in M$$ is a regular submanifold of $$E$$ of dimension $$k$$, I have to find a chart $$(A,\tau)$$ of the smooth manifold $$E$$ such that

$$\tau\big(A\cap\pi^{-1}(p)\big) = \tau(A)\cap)\big(\mathbb{R}^k \times\{c\}\big),$$

where $$c$$ is an element of $$\mathbb{R}^{d-k}$$ (typically $$c=0$$) and $$d=\mbox{dim }(E)$$.

So, my questions are:

1- Is right my proof?

2.- How I can see the fibre over $$p$$ is a regular submanifold? How proof that is diffeomorphic to $$F$$?

3.- If $$M$$ and $$F$$ are connected, then $$U\times F$$ is connected for all open set $$U\subset M$$ and thus the open set $$\pi^{-1}(U)$$ is connected in $$E$$ thanks to the diffeomorphism $$\Psi$$. So it is clear that $$E$$ is locally connected. However, the connectedness of $$E$$ has not proof yet. How is the proof?

• For 1st, yes. For 2nd, well, the part being diffeomorphic to F must be obvious, since $\Phi$ restricts to a desired diffeomorphism from $\pi^{-1}(p)$ to $\left\{p\right\}\times F \cong F$. being a regular submanifold can be achieved via the inverse function theorem, or by taking local charts of $U$ and $F$ and see it through $\Phi^{-1}$. And for the final one, I think using the fact that connectedness and path connectedness are equal on manifolds and finding a lifting of a path in $M$ work. – cjackal Oct 4 '17 at 0:27
• Yes, the diffeomorphism betwenn $\pi^{-1}(p)$ and $F$ was very stupid. I will try the other two. Thanks @cjackal – Dog_69 Oct 4 '17 at 15:22
• @Dog_69 : In details of your prove above, to show $\pi : E \rightarrow M$ is a submersion, i think you somehow confused about immersion and submersion maybe just typo.. – Sou Oct 31 '17 at 10:09
• @Dog_69 : i've edit that. Roll back if you disagree – Sou Nov 1 '17 at 2:55

I've been strugling with this exercise for a while too. Here is my solution. I agree with your solution to show that $\pi : E \rightarrow M$ is a submersion. To show that $E_p = \pi^{-1}(p)$ is regular submanifold diffeomorphic to $F$, we can use Theorem 3.5 in Jeffrey Lee's book. That is we just need to show that there is a smooth immersion homeomorphic to the fibre $E_p$.

$\textbf{Proof that$E_p$is a regular submanifold diffeomorphic to$F$}:$

Consider the diffeomorphism $\phi : \pi^{-1}(U) \rightarrow U \times F$. Note that $\{p\} \times F$ is a regular submanifold of $U\times F$. Because of this, the restriction of the smooth map $\phi^{-1}$ to $\{p\}\times F$ is smooth. That is we have a smooth map $$\phi^{-1}|_{\{p\}\times F} : \{p\}\times F \rightarrow \pi^{-1}(U)$$ where the image of the domain is $\pi^{-1}(p)=E_p$. Also because $\phi^{-1}$ is a diffeomorphism, then $d\phi^{-1}$ is an isomorphism for any point in $U\times F$. Therefore the differential of the map $\phi^{-1}|_{\{p\}\times F}$ is injective at each point. So the map is an immersion. Therefore $E_p$ is a regular submanifold diffeomorphic to $F$.

(Note :The diffeomorphic part of this conclusion does not explicitly state in the theorem 3.5 but we can prove that. I prefer the same theorem in other book such as John Lee's smooth manifold Proposition 5.2 which is include this.)

$\textbf{Proof that if$F$and$M$is connected, then so is$E$}$ :

To proof this i need this following theorem from topology (e.g from Willard's book) : If a topological space $X$ is connected and $\mathscr{U}$ is an open cover for $X$, then any two points can be connected by a simple chain consisting of elements of $\mathscr{U}$.

By local trivialization for each $p \in M$ we have an open subset $U \subset M$ containing $p$ and a diffeomorphism $\phi : \pi^{-1}(U) \rightarrow U \times F$. Let $\{\pi^{-1}(U)\}$ be the open cover for $E$. By above theorem we can have a simple chain connecting any two points $v,w \in E$ if all the elements of the open cover $\{\pi^{-1}(U)\}$ connected. Because $F$ and $M$ connected, then $U\subset M$ connected, $U \times F$ connected, $\phi^{-1}(U\times F) = \pi^{-1}(U)$ connected. So we have a simple chain where each of its elements is connected (implies path-connectedness). By this we can easily make a path connecting $v,w \in E$ by joining the paths from each chain.

I think many ways to prove this but this is the one that i find it quite convincing. Let me know if you have another solution or found error in my proof.

$\textbf{EDIT}$:

To see more clear that $E_p$ and $F$ is diffeomorphic, we can just restrict the map $\phi : \pi^{-1}(U) \rightarrow U \times F$ to the domain and codomain (which is both regular submanifold) $E_p$ and $\{p\} \times F$ respectively to obtain the map $$\phi|_{E_p}: E_p \rightarrow \{p\} \times F$$ Because the restriction of a smooth map to domain (or codomain) which is a regular submanifold is smooth, then the map above is a diffeomorphism. However, the alternative for the second proof (about connectedness of $E$) you can look at here Show that the total space $E$ of a fibre bundle $\pi : E \rightarrow M$ is connected.

Remark about restriction of smooth map to regular submanifold :

$\bullet$ Restriction to Domain

Let $F : M \rightarrow N$ be a smooth map and $S \subset M$ is a regular submanifold. Let $\iota : S \hookrightarrow M$ is the inclusion map. Then $F|_S = F \circ \iota : S \rightarrow N$ is smooth.

I know this result first from John Lee's book smooth manifold (p.112), but i didnt found it (explicitly) in Jeff Lee's book (it doesnt mean that its not there, because i'm not reading Jeff Lee's book thorough), maybe its because this result can easily proved. Here is the proof from John Lee's :

Notice that $S \subset M$ is a embedded submanifold (which is a regular submanifold in Jeff Lee's terminology) so by definition $\iota : S \hookrightarrow M$ is a smooth map. Therefor $F|_S = F \circ \iota : S \rightarrow N$ is a smooth map because its the composition of smooth maps.

Here the argument "$S\subset M \quad \text{reg. submanifold} \implies \iota : S \hookrightarrow M \quad \text{smooth}$ " follows from definition because John Lee define embedded submanifold (regular submanifold) very carefully from the beginning (look its definition on page 98). If you want to refer to Jeff Lee's book, he is mention it in p.132 (paragraph above Corollary 1.35). It states as

"... to say that inclusion $S \hookrightarrow M$ is an embedding (which is a smooth map) is easily seen to be the same that $S$ is a regular submanifold...".

But i dont think its really that easy for beginner to see. So i think the better way to study submanifold carefully is by John Lee's book. However i just found its direct proof in L.Tu's book Theorem 11.14

$\bullet$ Restriction to Codomain

The result for this available in John Lee's (p.113 Corollary 5.30) and Jeff Lee's books (p.132 Corollary 3.15). However, as usual, i prefer John Lee's. Here is what he says

Let $M$ be a smooth manifold and $S\subset M$ be an embedded submanifold (regular submanifold in Jeff Lee's). The every smooth map whose image contain in $S$ is also smooth as a map from $N$ to $S$.

By combining these two results, we can safely says that $$\phi|_{E_p}: E_p \rightarrow \{p\} \times F$$ is a diffeomorphism.

[Look Walter Poor's Differential Geometric Structure for the same proof (basically) except he use the Regular Level Set Theorem to show that $E_p = \pi^{-1}(p)$ is regular submanifold of $E$.]

• Your welcome. Please let me know if there's any query. – Sou Nov 4 '17 at 5:30
• First of all I'm sorry. I tried to answer you with my phone but it is stupid. And sorry again, I forgot this question. I didn't think someone answers me. I want to study your answer paciently, but it seems very good. And actually I have two questions: – Dog_69 Nov 6 '17 at 10:50
• 1.- First, you say that the restriction of a diffeomorphism to a regular submanifold is also a diffeomorphism. I didn't know this result. Where is it? In which book can I found it? What happens if I restrict it to any open set? Is it still an ismorphism? – Dog_69 Nov 6 '17 at 10:53
• 2.- This is a stupid question but, what do you mean when you said I'm quite confused about immersions and submersions? – Dog_69 Nov 6 '17 at 10:54
• Finally, thanks to mention John Lee's book and theorem, I will read the proof. And sorry for use four comments, but I don't know how write a new line in a comment (enter button and intro send the comment). Thanks @Sou – Dog_69 Nov 6 '17 at 10:57