Editing Summation (section)

===Special cases===
It is possible to sum fewer than 2 numbers:
* If the summation has one summand <math>x</math>, then the evaluated sum is <math>x</math>.
* If the summation has no summands, then the evaluated sum is [[0 (number)|zero]], because zero is the [[identity element|identity]] for addition. This is known as the ''[[empty sum]]''.

These degenerate cases are usually only used when the summation notation gives a degenerate result in a special case.
For example, if <math>n=m</math> in the definition above, then there is only one term in the sum; if <math>n=m-1</math>, then there is none.