This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A020490 #25 May 03 2022 11:04:10
%S A020490 1,2,3,4,6,8,10,12,18,24,30
%N A020490 Numbers k such that phi(k) <= sigma_0(k).
%C A020490 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 A020490 J.-M. De Koninck & A. Mercier, 1001 Problèmes en Théorie Classique des Nombres, Problème 750 pp. 95, 319-320, Ellipses Paris 2004.
%t A020490 Select[Range@ 1000000, EulerPhi@ # <= DivisorSigma[0, #] &] (* _Michael De Vlieger_, Oct 13 2015 *)
%o A020490 (PARI) isok(n) = eulerphi(n) <= numdiv(n); \\ _Michel Marcus_, Oct 13 2015
%Y A020490 Cf. A000005, A000010.
%K A020490 nonn,fini,full
%O A020490 1,2
%A A020490 _David W. Wilson_