The incomplete gamma function has a series expansion. This means that we're going to compute a sequence of values and then do a sum on those values. For more information, visit http://dlmf.nist.gov/8.
This series will have an infinite sequence of terms. Eventually, the values become too small to be relevant. We can establish a small value, ε, and stop the series when the next term is less than this value.
The calculation 
will yield alternating signs:
The sequence of terms looks like this with s=1 and ...
