OFFSET
0,1
FORMULA
a(n) ~ (1-r^4) * (2*n+1)! / r^(2*n+2), where r = 0.734095513758912755828782788976924944882810535913453055562... is the root of the equation arctanh(r) + arctan(r) = Pi/2. Also root of equation Pi + log((1-r)/(1+r)) = arctan(2*r/(1-r^2)). - Vaclav Kotesovec, Feb 07 2015
MATHEMATICA
nn = 20; Table[(CoefficientList[Series[Tan[ArcTan[x] + ArcTanh[x]], {x, 0, 2*nn+1}], x] * Range[0, 2*nn+1]!)[[n]], {n, 2, 2*nn, 2}] (* Vaclav Kotesovec, Feb 07 2015 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Patrick Demichel (patrick.demichel(AT)hp.com)
STATUS
approved