Definition:Summation/Notation

Notation for Summation
An indexed summation can also be denoted using an ellipsis:
 * $\displaystyle \sum_{i \mathop = a}^b f(i) = f(a) + f(a+1) + \cdots + f(b)$

The sign $\sum$ is called the summation sign and sometimes referred to as sigma (as that is its name in Greek).