OFFSET
0,2
FORMULA
a(n) ~ (2*n)! / r^(2*n+1), where r = 0.81654122617273381556779185061232051824097... is the root of the equation sec(r)*arctan(r) = 1. - Vaclav Kotesovec, Feb 05 2015
MATHEMATICA
nn = 20; Table[(CoefficientList[Series[ArcTanh[ArcTan[x]*Sec[x]], {x, 0, 2*nn+1}], x] * Range[0, 2*nn+1]!)[[n]], {n, 2, 2*nn, 2}] (* Vaclav Kotesovec, Feb 05 2015 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Patrick Demichel (patrick.demichel(AT)hp.com)
STATUS
approved