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In this case it is convenient to have some tools to check continuity of functions.
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\marginnote{For a proof refer to \cite[Proposition 7.1]{book:tu} or \cite[Theorem 3.70]{book:lee:topology}.}
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\begin{proposition}
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\marginnote{For a proof refer to \cite[Proposition 7.1]{book:tu} or \cite[Theorem 3.70]{book:lee:topology}.}
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Assume $F:X\to Y$ is a map between topological spaces and $\sim$ is an equivalence relation on $X$.
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Let $F$ be constant on each equivalence class $[p]\in X/\!\sim$, and denote $\widetilde F:X/\!\sim\to Y$, $\widetilde F([p]) := F(p)$ for $p\in X$, the map induced by $F$ on the quotient.
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Then, $\widetilde F$ is continuous if and only if $F$ is continuous.
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\end{proposition}
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Continuity of the projection implies that if $M/\!\sim$ is Hausdorff, then $\pi^{-1}(\pi(s)) = [s]$ is closed in $M$.
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Continuity of the projection $\pi: M \to M/\!\sim$implies that if $M/\!\sim$ is Hausdorff, then $\pi^{-1}(\pi(s)) = [s]$ is closed in $M$.
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If, additionally, $\pi$ is open\footnote{That is, it maps open sets to open sets.} then there is a stronger statement:
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\marginnote[4em]{These statements are not hard to prove, but their proofs will be omitted here.
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You can refer to~\cite[Chapters 7.1--7.5]{book:tu}.}
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\end{example}
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\begin{exercise}
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Show that there exists a diffeomorphism between the smooth structures$(\R, \id_\R)$ and $(\R, \psi)$from the previous example.
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Show that the smooth manifolds$(\R, \{(\R, \id_\R)\})$ and $(\R, \{(\R, \psi)\})$, defined using the smooth structures from the previous example, are diffeomorphic.
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