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Numbers k such that phi(k) <= sigma_0(k).
9

%I #25 May 03 2022 11:04:10

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

%N Numbers k such that phi(k) <= sigma_0(k).

%C 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

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

%t Select[Range@ 1000000, EulerPhi@ # <= DivisorSigma[0, #] &] (* _Michael De Vlieger_, Oct 13 2015 *)

%o (PARI) isok(n) = eulerphi(n) <= numdiv(n); \\ _Michel Marcus_, Oct 13 2015

%Y Cf. A000005, A000010.

%K nonn,fini,full

%O 1,2

%A _David W. Wilson_