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A012849
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arctanh(sec(x)*arctanh(x))=x+7/3!*x^3+193/5!*x^5+13319/7!*x^7...
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1, 7, 193, 13319, 1721281, 357281351, 108706678273, 45591739883975, 25211971076762753, 17775279702237604231, 15562493015528998511681, 16565188691481564795512967, 21067308092979783990029042241
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OFFSET
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0,2
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LINKS
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FORMULA
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a(n) ~ (2*n)! / r^(2*n+1), where r = 0.6588155299144662008044917663989656738607... is the root of the equation sec(r)*arctanh(r) = 1. - Vaclav Kotesovec, Feb 05 2015
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MATHEMATICA
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With[{nn=30}, Take[CoefficientList[Series[ArcTanh[Sec[x]ArcTanh[x]], {x, 0, nn}], x] Range[0, nn-1]!, {2, -1, 2}]] (* Harvey P. Dale, Jan 03 2014 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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Patrick Demichel (patrick.demichel(AT)hp.com)
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EXTENSIONS
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STATUS
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approved
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