# Map from Set of Continuous Functions on Interval to Set of their Integrals

## Theorem

Let $\Bbb I = \closedint a b$ be a closed interval on the set of real numbers $\R$ such that $a < b$.

Let $A$ denote the set of all continuous real functions $f: \Bbb I \to \R$.

Let $B \subseteq A$ denote the set of all functions differentiable on $\Bbb I$ whose derivative is continuous on $\Bbb I$.

Let $C \subseteq B$ denote the subset of $B$ which consists of all elements of $B$ such that $\map f a = 0$.

For each $f \in A$, let $h$ denote the mapping defined as:

- $\map {\paren {\map h f} } x = \displaystyle \int_a^x \map f t \rd t$

Then:

- $h: A \to C$

## Proof

Let $f \in A$ be an arbitrary continuous real function $f: \Bbb I \to \R$.

From Continuous Real Function is Darboux Integrable, $\map {\paren {\map h f} } x$ exists and is continuous on $\Bbb I$.

Let $x = a$.

Then we have:

\(\displaystyle \map {\paren {\map h f} } x\) | \(=\) | \(\displaystyle \int_a^x \map f t \rd t\) | Definition of $h$ | ||||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle \map {\paren {\map h f} } a\) | \(=\) | \(\displaystyle \int_a^a \map f t \rd t\) | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle 0\) | Definite Integral on Zero Interval |

Thus $h$ is a continuous real function on $\Bbb I$ such that $\map h a = 0$.

Further, by definition, $\int \map f t \rd t$ is the primitive of $f$, and exists by dint of the above.

It follows from the Fundamental Theorem of Calculus that $f = \map {\dfrac \d {\d x} } h$ and so $\map h x$ is functions differentiable on $\Bbb I$.

Thus it is shown that $C \subseteq B \subseteq A$ and the result follows.

$\blacksquare$

## Sources

- 1975: Bert Mendelson:
*Introduction to Topology*(3rd ed.) ... (previous) ... (next): Chapter $1$: Theory of Sets: $\S 9$: Inverse Functions, Extensions, and Restrictions: Exercise $1$