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A069924
Number of k, 1<=k<=n, such that phi(k) divides k.
1
1, 2, 2, 3, 3, 4, 4, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 9, 9, 10, 10, 10, 10, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 12, 12, 12, 12, 12, 12, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 14, 14, 14, 14, 14, 14, 14, 14, 15, 15, 15, 15, 15, 15, 15, 15
OFFSET
1,2
FORMULA
a(n) = Card(k: 1<=k<=n : k==0 (mod phi(k))) asymptotically: a(n) = C*log(n)^2 + o(log(n)^2) with C=0.6....
MATHEMATICA
Table[Length[Select[Range[n], Divisible[#, EulerPhi[#]] &]], {n, 1, 100}] (* Vaclav Kotesovec, Feb 16 2019 *)
Accumulate[Table[If[Divisible[n, EulerPhi[n]], 1, 0], {n, 80}]] (* Harvey P. Dale, Jul 04 2021 *)
PROG
(PARI) for(n=1, 150, print1(sum(i=1, n, if(i%eulerphi(i), 0, 1)), ", "))
CROSSREFS
Sequence in context: A085089 A228720 A219652 * A377028 A225559 A082479
KEYWORD
easy,nonn
AUTHOR
Benoit Cloitre, May 05 2002
STATUS
approved