Definition:Simple Function

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

A real-valued function $f: X \to \R$ is said to be a simple function it is a finite linear combination of characteristic functions:


 * $\ds f = \sum_{k \mathop = 1}^n a_k \chi_{S_k}$

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

Positive Simple Function
When all of the $a_i$ are positive, $f$ is also said to be positive.

Also known as
When it is desirable to emphasize the $\sigma$-algebra $\Sigma$, one also speaks of $\Sigma$-simple functions.

Also see

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