ArcSech[z] == (Sqrt[1/z - 1]/Sqrt[1 - 1/z]) (Pi/2 - Sqrt[1 - z^(-2)]/ (z (1 - (1 2)/(z^2 (3 - (1 2)/(z^2 (5 - (3 4)/(z^2 (7 - (3 4)/(z^2 (9 - (5 6)/(z^2 (11 - \[Ellipsis]))))))))))))) /; !IntervalMemberQ[Interval[{-1, 1}], z]

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2001-10-29