OFFSET
0,3
FORMULA
a(n) = (2*n)! * [x^(2*n)] sec(x*tan(x/2)).
EXAMPLE
sec(x*tan(x/2)) = 1 + 3*x^4/4! + 15*x^6/6! + 644*x^8/8! + ...
MATHEMATICA
nmax = 16; Table[(CoefficientList[Series[Sec[x Tan[x/2]], {x, 0, 2 nmax}], x] Range[0, 2 nmax]!)[[n]], {n, 1, 2 nmax + 1, 2}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Dec 22 2017
STATUS
approved