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%I #11 Feb 01 2022 09:42:02
%S 1,2,3,3,3,4,4,4,4,4,4,5,5,6,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,8,8,8,8,8,
%T 9,9,9,9,9,9,9,10,10,10,10,10,10,10,10,10,10,10,10,10,10,11,11,11,11,
%U 11,11,11,11,11,11,11,11,11,11,12,12,12,12,12,12,12,12,13,13,13
%N Number of balanced numbers <= n.
%C A balanced number k is a number such that phi(k) | sigma(k).
%F a(n) = Sum_{k<=n, phi(k)|sigma(k)} 1.
%F a(n) = Sum_{k=1..n} c(k), where c is the characteristic function of balanced numbers (A351114).
%e a(15) = 7; the 7 balanced numbers <= 15 are 1,2,3,6,12,14,15.
%t f[n_] := Boole[Divisible[DivisorSigma[1, n], EulerPhi[n]]]; Accumulate @ Array[f, 100] (* _Amiram Eldar_, Feb 01 2022 *)
%o (PARI) a(n) = sum(k=1, n, if (!(sigma(k) % eulerphi(k)), 1)); \\ _Michel Marcus_, Feb 01 2022
%Y Cf. A000010 (phi), A000203 (sigma), A020492 (balanced numbers), A351114.
%K nonn
%O 1,2
%A _Wesley Ivan Hurt_, Jan 31 2022
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