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 A125284 #4 Jul 21 2017 15:59:32
%S A125284 3,18,35,37,148,741,752,6814,13976,150095,517213,11874105
%N A125284 Lower indices of duplicate terms in A125204, i.e., k such that A125204(k) = A125204(k + 1).
%e A125284 18 is in the sequence because A125204(18) = A125204(19) = 266.
%t A125284 l = {0, 1}; Do[x = l[[n]] + l[[Mod[l[[n]], n] + 1]]; If[x == l[[n]], Print[n - 1]]; AppendTo[l, x], {n, 2, 10^9}]
%Y A125284 Cf. A125204.
%K A125284 hard,more,nonn
%O A125284 1,1
%A A125284 _Ryan Propper_, Jan 25 2007