User:Caliburn/s/8
Theorem
Let $\struct {X, \tau_1}$ be a connected topological space.
Let $\struct {Y, \tau_2}$ be a topological space.
Let $f : X \to Y$ be a locally constant function.
Then $f$ is constant.
Proof
Suppose that there exists some locally constant $f$ that is not constant.
That is, there at least two distinct points in $\map f X$.
We show that $X$ is then disconnected.
Let $y \in \map f X$.
Since $f$ is locally constant, for each $x \in X$ such that:
- $y = \map f x$
there exists an open set $U_x$ such that:
- $\map f {U_x} = \set y$
We now prove that:
- $\ds \map {f^{-1} } {\set y} = \bigcup_{x : y = \map f x} U_x$
For any $x \in \map {f^{-1} } {\set y}$, we have $\map f x = y$, so:
- $x \in U_x$
and hence:
- $\ds x \in \bigcup_{x : y = \map f x} U_x$
giving:
- $\ds \map {f^{-1} } {\set y} \subseteq \bigcup_{x : y = \map f x} U_x$
Now let:
- $\ds u \in \bigcup_{x : y = \map f x} U_x$
then:
- $u \in U_x$
for some $x$ with $y = \map f x$.
Since:
- $\map f {U_x} = \set y$
we then have:
- $\map f u = y$
so:
- $u \in \map {f^{-1} } {\set y}$
giving:
- $\ds \map {f^{-1} } {\set y} = \bigcup_{x : y = \map f x} U_x$
From the definition of a topological space, we have that:
So:
- $\map {f^{-1} } {\set y}$ is open for each $y \in \map f X$.
We therefore have:
\(\ds \bigcup_{y \in \map f X} \map {f^{-1} } {\set y}\) | \(=\) | \(\ds \map {f^{-1} } {\bigcup_{y \in \map f X} \set y}\) | Preimage of Union under Mapping: Family of Sets | |||||||||||
\(\ds \) | \(=\) | \(\ds \map {f^{-1} } {\map f X}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds X\) | Preimage of Mapping equals Domain |
Fix some $y_* \in \map f X$.
Since $\map f X$ contains at least two distinct points:
- $\map f X \setminus \set {y_*} \ne \O$
We then have:
- $\ds X = \map {f^{-1} } {\set {y_*} } \cup \bigcup_{y \in \map f X \setminus \set {y_*} } \map {f^{-1} } {\set y}$