when . This is read as "sum of , from to ". However, some notations may include the index at the upper bound of summation, or omit the index at the lower bound as in or , respectively. There are sigma notation variants where the range of bounds is omitted, which denotes the dummy variable only, like . Here is an example showing the summation of squares:

In general, while any variable can be used as the index of summation (provided that no ambiguity is incurred), some of the most common ones include letters such as , , , and ; the latter