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 A054251 #10 Aug 01 2019 09:00:05
%S A054251 1,0,1,1,2,1,5,1,9,7,19,1,45,1,83,79,165,1,417,1,827,639,1575,1,3875,
%T A054251 927,7025,5069,14689,1,35461,1,64199,47175,128399,52727,309767,1,
%U A054251 566565,409567,1186863,1,2835257,1,5202425,4888729,10357675,1,26066615
%N A054251 a(0) = 1; a(n) = Sum_{0 <= k < n and gcd(k, n) != 1} a(k).
%H A054251 G. C. Greubel, <a href="/A054251/b054251.txt">Table of n, a(n) for n = 0..1000</a>
%t A054251 a[0]:= 1; a[n_]:= a[n] =Sum[If[GCD[k, n]!=1, a[k], 0], {k,0,n-1}];
%t A054251 Table[a[n], {n, 0, 50}] (* _G. C. Greubel_, Jul 31 2019 *)
%o A054251 (PARI) a(n) = if(n==0, 1, sum(j=0,n-1, if(gcd(j,n)!=1, a(j), 0)));
%o A054251 vector(50, n, n--; a(n)) \\ _G. C. Greubel_, Jul 31 2019
%Y A054251 Cf. A045545.
%K A054251 nonn
%O A054251 0,5
%A A054251 _David W. Wilson_