# Pointwise Product of Measurable Functions is Measurable

## Theorem

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

Let $f, g: X \to \overline \R$ be $\Sigma$-measurable functions.

Then the pointwise product $f \cdot g: X \to \overline \R$ is also $\Sigma$-measurable.

## Proof

By Measurable Function is Pointwise Limit of Simple Functions, we find sequences $\sequence {f_n}_{n \mathop \in \N}, \sequence {g_n}_{n \mathop \in \N}$ such that:

- $\ds f = \lim_{n \mathop \to \infty} f_n$
- $\ds g = \lim_{n \mathop \to \infty} g_n$

where the limits are pointwise.

It follows that for all $x \in X$:

- $\map f x \cdot \map g x = \ds \lim_{n \mathop \to \infty} \map {f_n} x \cdot \map {g_n} x$

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so that we have the pointwise limit:

- $\ds f \cdot g = \lim_{n \mathop \to \infty} f_n \cdot g_n$

By Pointwise Product of Simple Functions is Simple Function, $f \cdot g$ is a pointwise limit of simple functions.

By Simple Function is Measurable, $f \cdot g$ is a pointwise limit of measurable functions.

Hence $f \cdot g$ is measurable, by Pointwise Limit of Measurable Functions is Measurable.

$\blacksquare$

## Sources

- 2005: René L. Schilling:
*Measures, Integrals and Martingales*... (previous) ... (next): $8.10$