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

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

Offset: 1

Author

David W. Wilson

Keywords

• nonn
• fini
• full

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.

Crossrefs

Cf. A000005, A000010.

Programs

Mathematica

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

PARI

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