Definition:Simple Function

Definition
Let $\left({X, \Sigma}\right)$ be a measurable space.

A real-valued function $f: X \to \R$ is said to be a simple function iff it is a finite linear combination of characteristic functions:
 * $\displaystyle f = \sum_{k=1}^n a_k \chi_{S_k}$

where $a_1, a_2, \ldots, a_n$ are real numbers and each of the sets $S_k$ is $\Sigma$-measurable.

Also see

 * Space of Simple Functions
 * Standard Representation of Simple Function
 * Simple Function is Measurable