OFFSET
1,2
FORMULA
E.g.f.: Sum_{k>=1} log(x^k + sqrt(1 + x^(2*k))).
a(n) = n! * Sum_{d|n, d odd} (-1)^((d - 1)/2) * ((d - 2)!!)^2 / d!.
MATHEMATICA
nmax = 21; CoefficientList[Series[Sum[ArcSinh[x^k], {k, 1, nmax}], {x, 0, nmax}], x] Range[0, nmax]! // Rest
Table[n! DivisorSum[n, (-1)^((# - 1)/2) ((# - 2)!!)^2/#! &, OddQ[#] &], {n, 1, 21}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Dec 16 2019
STATUS
approved