GammaRegularized[a, z] == 1 - z^a/(E^z Gamma[a])/ (a + ContinueFraction[{(-1)^k (a^((1/2) (1 - (-1)^k)) + Floor[(k - 1)/2]) z, a + k}, {k, 1, Infinity}]) /; !IntervalMemberQ[Interval[{-Infinity, 0}], z]

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2001-10-29