Mapping is Continuous iff Inverse Images of Open Sets are Open
Theorem
Let $X$ and $Y$ be normed vector spaces.
Let $f : X \to Y$ be a mapping.
Then:
- $f$ is continuous on $X$
Corollary
Let $X$ and $Y$ be normed vector spaces.
Let $f : X \to Y$ be a mapping.
Then:
- $f$ is continuous on $X$
Proof
Sufficient Condition
Let $c \in X$.
Let $\epsilon \in \R_{\mathop > 0}$.
Let $V := \map {B_\epsilon} {\map f c}$ be an open ball in $Y$.
By Open Ball is Open Set in Normed Vector Space, $V$ is an open set in $Y$.
Let $\map {f^{-1}} V = \map {f^{-1}} {\map {B_\epsilon} {\map f c}}$ be an open set in $X$.
Note that $\map f c \in \map {B_\epsilon} {\map f c}$.
Hence, $c \in \map {f^{-1}} {\map {B_\epsilon} {\map f c}}$.
By definition of open set:
- $\exists \delta > 0 : \map {B_\delta} c \subseteq \map {f^{-1}} {\map {B_\epsilon} {\map f c}}$
In other words:
- $\forall x \in X : \norm {x - c} < \delta \implies x \in \map {f^{-1}} {\map {B_\epsilon} {\map f c}}$
or
- $\map f x \in \map {B_\epsilon} {\map f c}$
By definition of open ball:
- $\norm {\map f x - \map f c} < \epsilon$
But $\epsilon$ was arbitrary.
Hence:
- $\forall \epsilon \in \R_{\mathop > 0} : \exists \delta > 0 : \norm {x - c} < \delta \implies \norm {\map f x - \map f c} < \epsilon$
Therefore, $f$ is continuous at $c$.
However, $c \in X$ was arbitrary.
Hence, $f$ is continuous on $X$.
$\Box$
Necessary Condition
Let $f$ be continuous on $X$.
Let $V$ be an open subset of $Y$.
Let $c \in \map {f^{-1}} V$.
Hence $\map f c \in V$.
By definition of an open set:
- $\exists \epsilon \in \R_{\mathop > 0} : \map {B_\epsilon} {\map f c} \subseteq V$
By continuity of $f$ at $c$:
- $\exists \delta \in \R_{\mathop > 0} : \forall x \in X : \norm {x - c} < \delta \implies \norm {\map f x - \map f c} < \epsilon$
In other words:
- $\exists \delta \in \R_{\mathop > 0} : \forall x \in X : \norm {x - c} < \delta \implies \map f x \in \map {B_\epsilon} {\map f c} \subseteq V$
or
- $\exists \delta \in \R_{\mathop > 0} : \forall x \in X : \norm {x - c} < \delta \implies \map f x \in V$
$\map f x \in V$ is equivalent to $x \in \map {f^{-1}} V$.
Suppose $\norm {x - c} < \delta$.
By definition of open ball:
- $x \in \map {B_\delta} c$
But then:
- $x \in \map {B_\delta} c \implies x \in \map {f^{-1}} V$
Hence:
- $\map {B_\delta} c \subseteq \map {f^{-1}} V$
But $\delta$ exists for any $c \in \map {f^{-1}} V$.
By definition, $\map {f^{-1}} V$ is open in $X$.
$\blacksquare$
Also see
Sources
- 2017: Amol Sasane: A Friendly Approach to Functional Analysis ... (previous) ... (next): Chapter $\S 2.2$: Continuous and linear maps. Continuous maps