I could have sworn I saw a question like this before, but I can't find it. Anyway:
Let $F: N \to M$ be a (continuous) smooth map from (topological) smooth $n$-manifold $N$ to (topological) smooth $m$-manifold $M$.
Question 1: Is this true?
$F$ is open (topological) smooth embedding if and only if injective local (homeomorphism) diffeomorphism.
Question 2 and Context for Question 1: $A$ is a smooth embedded $k$-submanifold of $M$, with $0 \le k \le m$, if and only if inclusion $\iota: A \to M$ is a smooth embedding. (This is either a theorem or a definition.) When we have $k=m$ itself, $\iota$ must surely be more than just a smooth embedding. I believe $\iota$ is an open smooth embedding, equivalently an injective local diffeomorphism. Is this right? And then for continuous case: upgrade topological embedding to open topological embedding, i.e. injective local homeomorphism. (See here or here.)
Definitions, Notes (I moved proof to answer):
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Definition of topological embedding := The induced map $\tilde F: N \to F(N)$ is a homeomorphism = $F$ is injective, continuous and open onto its image. = $\tilde F$ is injective, continuous and open.
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Definition 1 of smooth embedding := topological embedding + immersion
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Definition 2 of smooth embedding: $F(N)$ is smooth embedded $m$-submanifold of $M$, and then the induced map $\tilde F: N \to F(N)$ is diffeomorphism. (or say $F(N)$ is smooth embedded $k$-submanifold with $0 \le k \le m$ and then $k$ will turn out $k=m$.)
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Definition of local (homeo) diffeo: For all $p \in N$, there exists open subset $p \in U$ of $N$ s.t. $F(U)$ is open in $M$ and $F|_U: U \to F(U)$ is a (homeomorphism) diffeomorphism.
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Note 1: By '$F|_U: U \to F(U)$', I of course mean: Let $H=F|_U: U \to M$. Then consider the induced map $G=\tilde H: U \to H(U)=F(U)$. Then '$F|_U: U \to F(U)$' refers to $G$ instead of $H$.
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Note 2: I understand '$F|_U: U \to F(U)$ is a diffeomorphism.' to be equivalent to '$F|_U: U \to M$ is a smooth embedding.' (In re Note 1: $G$ is diffeomorphism if and only if $H$ is smooth embedding.)
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Note 3: I understand the following conditions to be equivalent:
- Condition 3.1. $F(U)$ is open in $M$.
- Condition 3.2. $F(U)$ is a smooth $k$-submanifold of $M$, and $n=m$.
- Condition 3.3. $F(U)$ is a smooth $k$-submanifold of $M$, and $F(N)$ is open in $M$.
- Condition 3.4. $F(N)$ is smooth $k$-submanifold of $M$, and then $F(U)$ is open in $F(N)$.
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Best Answer
For continuous case:
Local homeo is open onto its image: Open (see (1)) is actually stronger than open onto its image (and is equivalent to open onto its image + its image is open).
Open topological embedding is local homeo: Let $p \in N$. Choose $U = N$ itself.
For smooth case:
3.1. $\tilde F$ is open if and only if $F$ is open onto its image: '$F$ is open' is stronger than '$F$ is open onto its image' (and is equivalent to open onto its image + its image is open).
3.2. $\tilde F$ is injective since $F$ is injective.
3.3. $\tilde F$ is smooth: It's a rule that $F$ and $F(N)$ smooth $k$-submanifold (even if $k \ne m$) implies $\tilde F$ smooth.
3.4. $\tilde F^{-1}$ is smooth: It's a rule that this is equivalent to that $F$ is an immersion. It's part of proving the equivalence of Definitions 1 and 2.
Only if direction: