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A012152
arctanh(tan(tan(x)))=x+6/3!*x^3+176/5!*x^5+12240/7!*x^7...
0
1, 6, 176, 12240, 1561088, 318007808, 94807748608, 38942692972544, 21088169713729536, 14558790290337431552, 12481351528273319821312, 13009249833253480856289280, 16200881782506627598590672896
OFFSET
0,2
FORMULA
a(n) ~ (2*n)! / (arctan(Pi/4))^(2*n+1). - Vaclav Kotesovec, Feb 05 2015
MATHEMATICA
nn = 20; Table[(CoefficientList[Series[ArcTanh[Tan[Tan[x]]], {x, 0, 2*nn+1}], x] * Range[0, 2*nn+1]!)[[n]], {n, 2, 2*nn, 2}] (* Vaclav Kotesovec, Feb 05 2015 *)
CROSSREFS
Sequence in context: A024277 A012177 A012227 * A165928 A127910 A024278
KEYWORD
nonn
AUTHOR
Patrick Demichel (patrick.demichel(AT)hp.com)
STATUS
approved