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

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2001-10-29