Sigma

Sigma is the 18^th letter of the Greek alphabet, and we are talking about the capital version, . It signifies a summation, so that:

More generally,

Here are the different parts of sigma notation:

Figure 11.1 – A diagram of sigma notation

Here is an easy example to make the point better:

Variations

You will see shorthand in a lot of quantum computing books, but don't let it throw you. The following are all equivalent: