Pointwise Sum 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.

Assume that the pointwise sum $f + g: X \to \overline \R$ is well-defined.

Then $f + g$ is a $\Sigma$-measurable function.

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 $x \in X$:


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

so that we have the pointwise limit:


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

By Pointwise Sum of Simple Functions is Simple Function, $f + g$ is a pointwise limit of simple functions.

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

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