A012171 - OEIS

%I #9 Feb 05 2015 15:09:06

%S 1,2,16,496,24064,1908224,227958784,36620830720,7865201852416,

%T 2142236553248768,725477781101608960,298762982193334583296,

%U 146937839959786186080256,85124605007437662078894080

%N arctanh(tan(tanh(x)))=x+2/3!*x^3+16/5!*x^5+496/7!*x^7+24064/9!*x^9...

%F a(n) ~ (2*n)! / (arctanh(Pi/4))^(2*n+1). - _Vaclav Kotesovec_, Feb 05 2015

%t With[{nn=30},Take[CoefficientList[Series[ArcTanh[Tan[Tanh[x]]],{x,0,nn}], x] Range[0,nn-1]!,{2,-1,2}]] (* _Harvey P. Dale_, Nov 19 2011 *)

%K nonn

%O 0,2

%A Patrick Demichel (patrick.demichel(AT)hp.com)