OFFSET
1,2
FORMULA
a(n) = -(2*n)! * Sum_{d|n} A028296(d) / (2*d)!.
MATHEMATICA
nmax = 15; Table[(CoefficientList[Series[Sum[1 - Sech[x^k], {k, 1, nmax}], {x, 0, 2 nmax}], x] Range[0, 2 nmax]!)[[n]], {n, 1, 2 nmax + 1, 2}] // Rest
Table[-(2 n)! DivisorSum[n, EulerE[2 #]/(2 #)! &], {n, 1, 15}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Jan 14 2020
STATUS
approved