GamL(a, x) is the lower incomplete Gamma function with parameter a and argument x. It is defined as the Integral from 0 to x of exp(−t) * t^(a−1) dt (a > 0).
If x < a + 1 or x < 100,
is used, summed for n = 0 to when the sum stops changing.
If x >= a + 1 and x >= 100,
where GamU is the upper incomplete Gamma function. See the GamU(a, x) function.
Cannot take GamL(a, x) if a is an integer <= zero, except GamL(0, 0) = 0.
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