Summation Notation

Summation notation uses the Greek letter sigma ($\Sigma$) to represent adding up terms. You write $\sum_{i=1}^{n} a_i$ to mean $a_1 + a_2 + a_3 + ... + a_n$.

The $i = 1$ below sigma tells you where to start. The $n$ above sigma tells you where to stop. The expression after sigma tells you what to add.

For example, $\sum_{i=1}^{5} i = 1 + 2 + 3 + 4 + 5 = 15$. And $\sum_{i=1}^{4} 2^i = 2 + 4 + 8 + 16 = 30$.