Inverse of Homeomorphism is Homeomorphism
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Theorem
Let $T, T'$ be topological spaces.
Let $f: T \to T'$ be a homeomorphism.
Then $f^{-1}: T' \to T$ is also a homeomorphism.
Proof
By definition, a homeomorphism is a bijection such that both $f$ and $f^{-1}$ are continuous.
As $f$ is a bijection then by Bijection iff Inverse is Bijection, so is $f^{-1}$.
So by definition $f^{-1}$ is a bijection such that both $f^{-1}$ and $\left({f^{-1}}\right)^{-1}$ are continuous.
The result follows from Inverse of Inverse of Bijection.
$\blacksquare$