Summation notation, or "Sigma notation", is usually used to express sums whose summands are the terms in a sequence. The notation includes a lower bound and an upper bound that together identify the terms over which the sum occurs. The notation is used to represent both finite sums and infinite sums.
In the subscript beneath the Sigma can be found the index of the sum and the sum's lower limit. The lower limit identifies the starting term in our sequence of summands, and the upper limit identifies the final term.
The lower limit is often set to one, but it may be any number that is less than the upper limit. Moreover, the upper limit is often a number, but it may also be set to infinity. Here are a few examples:
The lower limit is 1
The upper limit is n
The lower limit is m
The upper limit is n
The lower limit is 1
The upper limit is infinity.
