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 A048866 #10 May 13 2017 03:43:04
%S A048866 1,1,0,0,0,0,0,-2,0,0,2,-2,2,-2,0,-2,4,-4,4,-4,0,-2,6,-6,4,-2,2,-2,10,
%T A048866 -6,10,-4,2,-2,6,-6,14,-2,4,-4,16,-8,16,-4,0,-2,18,-10,14,-6,6,-2,22,
%U A048866 -10,12,-4,8,0,26,-12,26,-2,4,-2,16,-10,30,-2,10,-8,32,-12,32,-2,2,-2
%N A048866 Difference between number of nonprimes and primes in reduced residue system of n.
%H A048866 G. C. Greubel, <a href="/A048866/b048866.txt">Table of n, a(n) for n = 1..1000</a>
%F A048866 a(n) = A000010(n) - 2*A000720(n) + 2* A001221(n).
%e A048866 n = 31: Phi(31) = 30 from which 10 is prime and 20 is not: a(31) = 20-10 = 10;
%e A048866 n = 20: Phi(20) = 8 from which 6 are primes and 2 are not: a(20) = 2-6 = -4;
%e A048866 n = 15: reduced residue system = {1,2,7,4,8,11,13,14}, primes = {2,7,11,13,}, nonprimes = {2,4,8,14}, thus a(15) = 4-4 = 0.
%t A048866 Table[EulerPhi[n] - 2*PrimePi[n] + 2*PrimeNu[n] , {n,1,50}] (* _G. C. Greubel_, May 12 2017 *)
%Y A048866 Cf. A000010, A000720, A001221.
%K A048866 sign
%O A048866 1,8
%A A048866 _Labos Elemer_