OFFSET
1,2
FORMULA
a(n) = Sum_{d|n, phi(d)|n} 1.
MATHEMATICA
a[n_] := DivisorSum[n, 1 &, Divisible[n, EulerPhi[#]] &]; Array[a, 100] (* Amiram Eldar, Apr 22 2023 *)
PROG
(PARI) a(n) = sumdiv(n, d, !(n % eulerphi(d))); \\ Michel Marcus, Apr 22 2023
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wesley Ivan Hurt, Apr 21 2023
STATUS
approved