# Embedded Submanifolds Have a Unique Smooth Structure

Let $M$ be a smooth manifold. An embedded submanifold of $M$ is a subset $S$ of $M$ such that $S$ is a topological manifold under the subspace topology induced by $M$, endowed with a smooth structure such that the inclusion map $i:S\to M$ is a smooth embedding.

Is it true that if $M$ is a smooth manifold and $S$ is a subset $M$, then there is a unique smooth structure, if one exists, which makes $S$ into an embedded submanifold?

Suppose there is a smooth structure on S which makes the inclusion map $i:S\to M$ a smooth embedding. Let $\mathscr A =\{(U_\alpha,\varphi_\alpha)\}_{\alpha\in J}$ be a smooth atlas on $M$. Then I think the set $$\mathscr B=\{(U_\alpha\cap S,\varphi_\alpha|_{U_\alpha\cap S}):\alpha\in J\}$$ is a smooth atlas on $S$, since any two elments in $\mathscr B$ are smoothly compatible (borrowing from the smooth compatibility of $(U_\alpha,\varphi_\alpha)$ and $(U_\beta,\varphi_\beta)$). The inclusion map can also be shown to be smooth under this smooth structure. Since any smooth atlas is contained in a unique maximal smooth atlas, there is a unique smooth strucure on $S$.

Is this reasoning correct?

• Lee's book (page 114) for the uniqueness of the topology/smooth structures on $S$ that makes $i$ an embedding. – jimbo Jan 22 '15 at 2:45
• @jimbo Your response certainly helped. Can you also see if my working above is correct? – caffeinemachine Jan 22 '15 at 3:16

## 1 Answer

Let $p\in{S}$, $(\phi, U)$ such $\phi(S\cap{U})=\phi(U)\cap(\mathbb{R}^n\times{0})$

$$\phi\circ{i_S}\circ(\phi|_{S\cap{U}})^{-1}:\phi(U)\cap(\mathbb{R}^n\times{0})\to\phi(U)\subset\mathbb{R}^n$$ is the inclusion map, hence smooth.

In fact it´s a local representation of $i_S$. Then induced structure por $M$ is the same as the original structure of $S$.