OFFSET
0,2
FORMULA
a(n) ~ (2*n)! * sinh(2*r) / ((Pi/2 + (sinh(r))^2/(1-r^2)) * r^(2*n+1)), where r = 0.970313828446324195188532490988552661547225320103102781254845... is the root of the equation arctanh(r)*tanh(r) = Pi/2. - Vaclav Kotesovec, Feb 06 2015
MATHEMATICA
nn = 20; Table[(CoefficientList[Series[Tan[ArcTanh[x]*Tanh[x]], {x, 0, 2*nn}], x] * Range[0, 2*nn]!)[[n]], {n, 1, 2*nn+1, 2}] (* Vaclav Kotesovec, Feb 06 2015 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Patrick Demichel (patrick.demichel(AT)hp.com)
EXTENSIONS
a(0)=0 prepended by Vaclav Kotesovec, Feb 06 2015
STATUS
approved