Complex GamU(a, x) = Upper Incomplete Gamma Function
If a.i == 0 and x.i == 0 and a.r != −int and x.r >= 0, use the real GamU(a.r, x.r) function.
Else if x = 0,
If a is a real integer <= zero,
"Cannot take GamU(a, 0) if a is an integer <= zero".
Else GamU(a, 0) = Gam(a).
Else if a is not a real integer <= zero and (x < a + 1 or x < 100),
compute GamL(a, x) by series, Then GamU(a, x) = Gam(a) − GamL(a, x).
Else Compute GamU(a, x) by continued fraction method.
See: Incomplete Gamma Function -- From MathWorld
And: Wolfram Function Evaluation -- Gamma2 (GamU(a, x) = Gamma[a, x])
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