OFFSET
0,3
FORMULA
a(n) ~ (n-1)! / (2 * r^n), where r = log((1 + (19 - 3*sqrt(33))^(1/3) + (19 + 3*sqrt(33))^(1/3))/3) = 0.60937786343600623153680337116839869542853927931... is the root of the equation exp(r)-sech(r) = 1. - Vaclav Kotesovec, Feb 05 2015
MATHEMATICA
CoefficientList[Series[ArcTanh[Sinh[x]*(1 + Tanh[x])], {x, 0, 20}], x] * Range[0, 20]! (* Vaclav Kotesovec, Feb 05 2015 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Patrick Demichel (patrick.demichel(AT)hp.com)
EXTENSIONS
Prepended missing a(0)=0 from Vaclav Kotesovec, Feb 05 2015
STATUS
approved