OFFSET
1,2
COMMENTS
A balanced number k is a number such that phi(k) | sigma(k).
FORMULA
a(n) = Sum_{k<=n, phi(k)|sigma(k)} 1.
a(n) = Sum_{k=1..n} c(k), where c is the characteristic function of balanced numbers (A351114).
EXAMPLE
a(15) = 7; the 7 balanced numbers <= 15 are 1,2,3,6,12,14,15.
MATHEMATICA
f[n_] := Boole[Divisible[DivisorSigma[1, n], EulerPhi[n]]]; Accumulate @ Array[f, 100] (* Amiram Eldar, Feb 01 2022 *)
PROG
(PARI) a(n) = sum(k=1, n, if (!(sigma(k) % eulerphi(k)), 1)); \\ Michel Marcus, Feb 01 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Wesley Ivan Hurt, Jan 31 2022
STATUS
approved