OFFSET
0,2
COMMENTS
Invert transform of A000031.
FORMULA
a(0) = 1; a(n) = Sum_{k=1..n} A000031(k) * a(n-k).
MATHEMATICA
nmax = 27; CoefficientList[Series[1/(1 + Sum[EulerPhi[k] Log[1 - 2 x^k]/k, {k, 1, nmax}]), {x, 0, nmax}], x]
a[0] = 1; a[n_] := a[n] = Sum[(1/k) DivisorSum[k, EulerPhi[#] 2^(k/#) &] a[n - k], {k, 1, n}]; Table[a[n], {n, 0, 27}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Nov 11 2019
STATUS
approved