TITLE: What does the Greek "E"-like symbol mean?
QUESTION [6 upvotes]: I have a simple question: what does the following mean?
$$\displaystyle\sum_{k=0}^n$$

REPLY [8 votes]: This symbol is a Greek letter called (capital) sigma. It's used to denote sums. The definition is
$$
\sum_{i=1}^n a_i = a_1+\cdots+a_n.
$$
For example:
$$
\sum_{i=1}^6 1 = \underbrace{1+\cdots+1}_{\text{6 times}} = 6
$$
or 
$$
\sum_{i=1}^4 i = 1 + 2 +3+4 = 10.
$$