OFFSET
0,2
FORMULA
a(n) ~ (2*n)! / (tanh(Pi/4))^(2*n+1). - Vaclav Kotesovec, Feb 05 2015
MAPLE
a:= n-> (t-> t!*coeff(series(arctanh(tan(arctanh(x))), x, t+1), x, t))(2*n+1):
seq(a(n), n=0..15); # Alois P. Heinz, Aug 17 2018
MATHEMATICA
nn = 20; Table[(CoefficientList[Series[ArcTanh[Tan[ArcTanh[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