ArcCosh[z] == (Sqrt[z - 1]/Sqrt[1 - z]) (Pi/2 - z (Sqrt[1 - z^2]/(1 + ContinueFraction[ {-2 (2 Floor[(k + 1)/2] - 1) Floor[(k + 1)/2] z^2, 2 k + 1}, {k, 1, Infinity}]))) /; !IntervalMemberQ[Interval[{-Infinity, -1}], z] && !IntervalMemberQ[Interval[{1, Infinity}], z]

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2001-10-29