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 A304576 #9 May 15 2018 08:32:49
%S A304576 1,1,0,2,1,2,1,4,2,3,1,4,2,5,2,7,3,6,4,7,4,9,5,8,5,10,3,9,5,8,5,13,5,
%T A304576 13,5,11,7,15,5,14,8,11,8,17,6,18,8,15,8,17,7,19,10,16,9,20,9,23,12,
%U A304576 15,13,25,8,26,10,18,13,26,11,22,14,22,15,30,9,29
%N A304576 a(n) = Sum_{k < n, k squarefree and relatively prime to n} (-1)^(k-1).
%t A304576 l[n_]:=Sum[(-1)^(k-1),{k,Select[Range[n],And[SquareFreeQ[#],GCD[n,#]==1]&]}];
%t A304576 Table[l[n],{n,100}]
%o A304576 (PARI) a(n) = sum(k=1, n, if (issquarefree(k) && (gcd(n,k)==1), (-1)^(k-1))); \\ _Michel Marcus_, May 15 2018
%Y A304576 Cf. A000010, A001221, A005117, A008683, A073311, A111972, A112399, A139555, A265675, A304573, A304574, A304575.
%K A304576 nonn
%O A304576 1,4
%A A304576 _Gus Wiseman_, May 14 2018