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 A253590 #15 Jan 26 2025 09:12:41
%S A253590 1,2,10,39,229,2484,31775,532977,10038368,229479450
%N A253590 Indices of the primorial numbers in A098550.
%F A253590 A098550(a(n)) = A002110(n).
%t A253590 a = {1, 2, 3}; NN = Range[4, 12000]; g = {-1}; While[g[[1]] != 0, g = Flatten[{FirstPosition[NN, v_ /; GCD[a[[-1]], v] == 1 && GCD[a[[-2]], v] > 1, 0]}]; If[g[[1]] != 0, d = NN[[g]]; a = Flatten[Append[a, d[[1]]]]; NN = Delete[NN, g[[1]]]]]; a002110[n_] := Product[Prime[j], {j, n}]; Flatten[Table[Position[a, k_ /; k == a002110[n]], {n, 0, 5}]]
%Y A253590 Cf. A002110 (primorial numbers), A098550.
%K A253590 nonn,hard,more
%O A253590 0,2
%A A253590 _L. Edson Jeffery_, Jan 04 2015
%E A253590 a(0) prepended by and a(6)-a(9) from _Jinyuan Wang_, Jan 26 2025