Equivalence of Definitions of Homeomorphic Metric Spaces

Theorem
Let $M_1 = \struct {A_1, d_1}$ and $M_2 = \struct {A_2, d_2}$ be metric spaces.

Let $f: A_1 \to A_2$ be a bijection.

Proof
In order to prove the assertion it is sufficient to prove that the conditions for homeomorphism in definitions $2$ to $4$ are necessary and sufficient conditions for $f$ and $f^{-1}$ to be continuous on $M_1$ and $M_2$ respectively.

Definition 1 implies Definition 2
Let $f$ and $f^{-1}$ both be continuous by definition $2$.

Let $U \subseteq A_1$ be open in $M_1$.

As $f^{-1}$ is continuous, $\paren {f^{-1} }^{-1} \sqbrk U = f \sqbrk U$ is open in $M_2$.

That is, $f \sqbrk U = f \sqbrk U$ is open in $M_2$.

Let $f \sqbrk U \subseteq A_2$ be open in $M_2$.

Then as $f$ is continuous, $f^{-1} \sqbrk {f \sqbrk U} = U$ is open in $M_1$.

Thus:
 * for all $U \subseteq A_1$, $U$ is an open set of $M_1$ $f \sqbrk U$ is an open set of $M_2$.

Definition 2 implies Definition 4
Let definition $2$ hold.

Let $a \in A_1$.

Let $N \subseteq A_1$.

Then:
 * $N$ is a neighborhood of $a$


 * $N$ contains an open set $U$ containing $a$
 * $N$ contains an open set $U$ containing $a$


 * $f \sqbrk N$ contains an open set $U' = f \sqbrk U$ containing $\map f a$
 * $f \sqbrk N$ contains an open set $U' = f \sqbrk U$ containing $\map f a$


 * $f \sqbrk N$ is a neighborhood $\map f a$.
 * $f \sqbrk N$ is a neighborhood $\map f a$.

Definition 4 implies Definition 1
Let definition $4$ hold.

Let $a \in A_1$.

Let $U \subseteq A_2$ be a neighborhood of $\map f a$.

Then $f \sqbrk {f^{-1} \sqbrk U}$ is a neighborhood $\map f a$.

Hence $f^{-1} \sqbrk U$ is a neighborhood of $a$.

Thus $f$ is continuous.

Similarly, let $b \in A_2$.

Let $V \subseteq A_1$ be a neighborhood of $\map {f^{-1} } b$.

Then $f^{-1} \sqbrk {f \sqbrk V}$ is a neighborhood $\map {f^{-1} } b$.

Hence $f \sqbrk V$ is a neighborhood of $\map f {\map {f^{-1} } b} = b$.

Thus $f^{-1}$ is continuous.

Definition 2 iff Definition 3
This is demonstrated in Continuity of Mapping between Metric Spaces by Closed Sets.