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 A225680 #17 Feb 22 2021 12:04:27 %S A225680 1,2,3,5,3,7,5,11,13,7,15,17,19,7,11,23,13,29,15,31,33,17,35,37,19,13, %T A225680 41,7,43,23,47,51,53,11,19,29,59,61,31,65,33,67,69,35,71,73,37,77,13, %U A225680 79,41,83,85,43,87,89,91,31,47,95,97,101,51,103,35,53,107 %N A225680 Denominators of phi(k)/k, as k runs through the squarefree numbers (A005117). %C A225680 To every fraction taken by the arithmetical function m -> phi(m)/m there is exactly one n such that A225679(n)/A225680(n) is equal to it. %H A225680 Ivan Neretin, <a href="/A225680/b225680.txt">Table of n, a(n) for n = 1..10000</a> %F A225680 a(n)=A005117(n)/gcd(A000010(A005117(n)),A005117(n)). %e A225680 A005117(5)=6, phi(6)/6=2/6=1/3, so a(5)=3. %t A225680 s = Select[Range[200], SquareFreeQ]; Denominator[EulerPhi[s]/s] (* _T. D. Noe_, May 13 2013 *) %o A225680 (PARI) lista(nn) = apply(x->(denominator(eulerphi(x)/x)), Vec(select(issquarefree, [1..nn], 1))); \\ _Michel Marcus_, Feb 22 2021 %Y A225680 Cf. A225679 (numerators). %K A225680 nonn,frac,easy %O A225680 1,2 %A A225680 _Franz Vrabec_, May 12 2013