Incomplete gamma functions are given by:

         /x          a-1
P(a,x) = |  exp(-t) t    dt
         /0

and

         /inf          a-1
Q(a,x) = |    exp(-t) t    dt
         /x

for a > 0. It is easy to see that Gamma(a) = P(a,x) + Q(a,x).

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