OFFSET
0,2
FORMULA
a(n) = (2*n+1)! * [x^(2*n+1)] arctanh(x*sec(x)).
EXAMPLE
arctanh(x*sec(x)) = x/1! + 5*x^3/3! + 109*x^5/5! + 5977*x^7/7! + 612729*x^9/9! + ...
MATHEMATICA
nmax = 15; Table[(CoefficientList[Series[ArcTanh[x Sec[x]], {x, 0, 2 nmax + 1}], x] Range[0, 2 nmax + 1]!)[[n]], {n, 2, 2 nmax, 2}]
PROG
(PARI) first(n) = x='x+O('x^(2*n)); vecextract(Vec(serlaplace(atanh(x/cos(x)))), (4^n - 1)/3) \\ Iain Fox, Dec 19 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Dec 19 2017
STATUS
approved