OFFSET
0,2
FORMULA
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
MATHEMATICA
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 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Patrick Demichel (patrick.demichel(AT)hp.com)
EXTENSIONS
Definition clarified by Harvey P. Dale, Jan 03 2014
STATUS
approved