GammaRegularized[a, z] == 1 - (z^a/(E^z Gamma[a])) (1/(a - (a z)/(a + 1 + z/(a + 2 - ((a + 1) z)/(a + 3 + (2 z)/(a + 4 - ((a + 2) z)/(a + 5 + \[Ellipsis]))))))) /; !IntervalMemberQ[Interval[{-Infinity, 0}], z]

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2001-10-29