A020490 Numbers k such that phi(k) <= sigma_0(k).

1, 2, 3, 4, 6, 8, 10, 12, 18, 24, 30

Offset

1,2

Comments

The positive integers k such that 2^phi(k) <= 2*k form the subsequence {1, 2, 3, 4, 6, 8, 10, 12} (De Koninck & Mercier). - Bernard Schott, May 02 2022

References

J.-M. De Koninck & A. Mercier, 1001 Problèmes en Théorie Classique des Nombres, Problème 750 pp. 95, 319-320, Ellipses Paris 2004.

Links

Table of n, a(n) for n=1..11.

Mathematica

Select[Range@ 1000000, EulerPhi@ # <= DivisorSigma[0, #] &] (* Michael De Vlieger, Oct 13 2015 *)

Prog

(PARI) isok(n) = eulerphi(n) <= numdiv(n); \\ Michel Marcus, Oct 13 2015

Crossrefs

Cf. A000005, A000010.

Sequence in context: A067128 A245779 A120432 * A217660 A275581 A014875

Adjacent sequences: A020487 A020488 A020489 * A020491 A020492 A020493

Keyword

nonn,fini,full

Author

David W. Wilson

Status

approved