GamU(a, x) is the upper incomplete Gamma function with parameter a and argument x. It is defined as the Integral from x to infinity of exp(−t) * t^(a−1) dt (a > 0).

If x < a + 1 or x < 100,

where GamL is the Lower incomplete Gamma function. See the GamL(x, y) function.

If x >= a + 1 and x >= 100,

is used, summed evaluated until the value stops changing. The notation (1/x+ (a−1)/1+ 1/x+ (2−a)/1+ 2/x+ ...) represents a continued fraction development for GamU(a, x) that converges for x > 0.

Cannot take GamU(a, 0) if a is an integer <= zero.

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