Summation notation allows an expression that contains a sum to be expressed in a simple, compact manner. The uppercase Greek letter sigma, Σ, is used to denote the sum of a set of numbers.
- Example

Let
be a function and
are integers with
. Then
.
We say
is the lower limit and
is the upper limit of the sum.
We can replace the letter
with any other variable. For this reason
is referred to as a dummy variable. So...

Conventionally we use the letters
,
,
,
for dummy variables.
- Example

Here, the dummy variable is
, the lower limit of summation is 1, and the upper limit is 5.
- Example
Sometimes, you will see summation signs with no dummy variable specified, e.g.,

In such cases the correct dummy variable should be clear from the context.
You may also see cases where the limits are unspecified. Here too, they must be deduced from the context.