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 A228940 #15 Sep 14 2013 20:01:31
%S A228940 1,2,3,5,13,16,21,29,34,160,178,231,281,416,466,816,866,1131,1181,
%T A228940 25431,25769,31515,31853,53105,53617,78705,79217,172615,173497,216715,
%U A228940 217597,665015,666697,749115,750797,1278005,1280317,1393605,1395917,646710161
%N A228940 Nodes of tree generated as follows: (1,2) is an edge, and if (x,y) is an edge, then (y, y^2 - x^2) and (y, y^2 + x^2) are edges.
%H A228940 Vincenzo Librandi, <a href="/A228940/b228940.txt">Table of n, a(n) for n = 1..1000</a>
%e A228940 Taking the first generation of edges to be G(1) = {(1,2)}, the edge (1,2) grows G(2) = {(2,3), (2,5)}, which grows G(3) = {(3,5), (3,14), (5,21), (5,29)}, ... Expelling duplicate nodes and sorting leave (1, 2, 3, 5, 13, 16, 21, 29, 34, 160,...).
%t A228940 f[x_, y_] := {{y, y^2-x^2}, {y, y^2 + x^2}}; x = 1; y = 2; t = {{x, y}};
%t A228940 u = Table[t = Flatten[Map[Apply[f, #] &, t], 1], {15}]; v = Flatten[u];
%t A228940 w = Flatten[Prepend[Table[v[[2 k]], {k, 1, Length[v]/2}], {x, y}]];
%t A228940 Sort[Union[w]]
%Y A228940 Cf. A228853, A228940.
%K A228940 nonn,easy
%O A228940 1,2
%A A228940 _Clark Kimberling_, Sep 08 2013