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 A064020 #18 Dec 15 2017 17:35:36 %S A064020 2,3,9,33,363,1089,1653,3237,4297761,32348343651 %N A064020 Numbers n such that sf(n) = phi(n), where sf(n) = A013928(n). %C A064020 No more terms < 30000. All terms, except the first, are multiples of 3. - _Emeric Deutsch_, Jun 25 2005 %C A064020 a(11) > 5*10^10. - _Donovan Johnson_, Jun 10 2011 %C A064020 No other terms < 10^12. - _Jud McCranie_, Aug 27 2013 %e A064020 sf(363) = 220, phi(363) = 220. %p A064020 with(numtheory): a:=proc(n) if add(mobius(k)^2,k=1..n-1)=phi(n) then n else fi end: seq(a(n),n=1..5000); # _Emeric Deutsch_, Jun 25 2005 %o A064020 (PARI) sf(n)=s=0; for(x=1,n, if(issquarefree(x),s++)); s; for(n=1,10^5, if(sf(n)==eulerphi(n),print(n))) %K A064020 more,nonn %O A064020 1,1 %A A064020 _Jason Earls_, Sep 07 2001 %E A064020 Corrected and extended by _Emeric Deutsch_, Jun 25 2005 %E A064020 a(9) from _Donovan Johnson_, Jul 15 2009 %E A064020 a(10) from _Donovan Johnson_, Jun 10 2011