ArcTanh[z] == z/(1 - z^2 + (2 z^2)/(3 + (2 z^2)/(5 (1 - z^2) + (12 z^2)/(7 + (12 z^2)/(9 (1 - z^2) + (30 z^2)/ (11 + (30 z^2)/(13 (1 - z^2) + \[Ellipsis]))))))) /; !IntervalMemberQ[Interval[{-Infinity, -1}], z] && !IntervalMemberQ[Interval[{1, Infinity}], z]

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2001-10-29