# Pointwise Sum of Simple Functions is Simple Function

## Contents

## Theorem

Let $\struct {X, \Sigma}$ be a measurable space.

Let $f, g : X \to \R$ be simple functions.

Then $f + g: X \to \R, \map {\paren {f + g} } x := \map f x + \map g x$ is also a simple function.

## Proof

We have $f + g = + \circ \innerprod f g \circ \Delta_X$, where:

- $\Delta_X: X \to X \times X$ is the diagonal mapping on $X$
- $\innerprod f g: X \times X \to \R \times \R, \map {\innerprod f g} {x, y} := \tuple {\map f x, \map g y}$
- $+: \R \times \R \to \R$ is real addition.

From this, we see that $+$ may be restricted to $\Img {\innerprod f g}$, the image of $\sequence {f, g}$.

It is immediate that this image equals $\Img f \times \Img g$.

By Measurable Function is Simple Function iff Finite Image Set, $\Img f$ and $\Img g$ are finite.

Therefore, by Cardinality of Cartesian Product, $\Img {\sequence {f, g} }$ is also finite.

Next, from Corollary to Image of Composite Mapping:

- $\Img {\innerprod f g \circ \Delta_X} \subseteq \Img {\innerprod f g}$

whence the former is finite.

Now $+$ is a surjection:

- $+: \Img {\innerprod f g \circ \Delta_X} \to \Img {f + g}$

by Restriction of Mapping to Image is Surjection.

By Cardinality of Surjection, this implies that $\Img {f + g}$ is finite.

Whence Measurable Function is Simple Function iff Finite Image Set grants that $f + g$ is a simple function.

$\blacksquare$

## Also see

- Definition:Pointwise Operation, of which the definition of $f + g$ is an instantiation
- Scalar Multiple of Simple Function is Simple Function
- Pointwise Product of Simple Functions is Simple Function
- Space of Simple Functions forms Algebra

## Sources

- 2005: René L. Schilling:
*Measures, Integrals and Martingales*... (previous) ... (next): $8.7 \ \text{(iv)}$