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 A066087 #16 Oct 18 2019 00:07:15
%S A066087 0,0,0,0,0,0,0,0,-1,0,0,2,0,0,0,0,0,1,0,0,0,0,0,2,-1,0,0,-2,0,0,0,0,0,
%T A066087 0,0,-1,0,0,0,0,0,0,0,2,-2,0,0,2,1,-1,0,-4,0,4,0,18,0,0,0,0,0,0,0,0,0,
%U A066087 0,0,0,0,0,0,1,0,0,-4,-2,0,0,0,0,-1,0,0,-4,0,0,0,18,0,-2,0,2,0,0,0,2,0,-3,8,-1,0,0,0,0,0,0,0,2,0,0,0,2,0,0,0,12,-6
%N A066087 a(n) = gcd(sigma(n), phi(n)) - gcd(sigma(rad(n)), phi(rad(n))); rad = A007947.
%H A066087 Charles R Greathouse IV, <a href="/A066087/b066087.txt">Table of n, a(n) for n = 1..10000</a>
%F A066087 A009223(n) - A066086(n) = gcd(sigma(n), phi(n)) - gcd(sigma(A007947(n)), phi(A007947(n))).
%t A066087 Table[GCD[DivisorSigma[1, n], EulerPhi@ n] - GCD[DivisorSigma[1, #], EulerPhi@ #] &[Times @@ FactorInteger[n][[All, 1]]], {n, 120}] (* _Michael De Vlieger_, Feb 19 2017 *)
%o A066087 (PARI) rad(f)=for(i=1,#f~,f[i,2]=1); f
%o A066087 g(f)=gcd(sigma(f),eulerphi(f))
%o A066087 a(n)=my(f=factor(n),k=rad(f)); g(f)-g(k) \\ _Charles R Greathouse IV_, Dec 09 2013
%Y A066087 Cf. A048250, A023900, A000203, A007947, A000010, A009223, A066086.
%K A066087 sign
%O A066087 1,12
%A A066087 _Labos Elemer_, Dec 04 2001