OFFSET
1,4
FORMULA
G.f. A(x) satisfies: A(x) = x + x^2 * ( 1 + Sum_{k>=1} phi(k) * A(x^k) ).
a(1) = a(2) = 1; a(n+2) = Sum_{d|n} phi(n/d) * a(d).
MATHEMATICA
a[1] = a[2] = 1; a[n_] := a[n] = Sum[a[GCD[n - 2, k]], {k, 1, n - 2}]; Table[a[n], {n, 1, 55}]
nmax = 55; A[_] = 0; Do[A[x_] = x + x^2 (1 + Sum[EulerPhi[k] A[x^k], {k, 1, nmax}]) + O[x]^(nmax + 1) // Normal, nmax + 1]; CoefficientList[A[x], x] // Rest
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, May 07 2024
STATUS
approved