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 A228939 #11 Sep 14 2013 20:01:18
%S A228939 1,2,4,5,8,10,16,20,29,32,50,68,80,125,128,145,256,320,416,500,544,
%T A228939 640,866,1088,1250,1600,2048,2600,4205,4688,5120,6464,6800,8192,8320,
%U A228939 15725,16640,21866,25000,25114,34816,36992,51200,66560,102656,128000,130000
%N A228939 Nodes of tree generated as follows: (1,2) is an edge, and if (x,y) is an edge, then (y,x*y) and (y,x^2 + y^2) are edges.
%e A228939 Taking the first generation of edges to be G(1) = {(1,2)}, the edge (1,2) grows G(2) = {(2,2), (2,5)}, which grows G(3) = {(2,4), (2,8), (5,10), (5,29)}, ... Expelling duplicate nodes and sorting leave (1, 2, 4, 5, 8, 10, 16, 20, 29, 32,...).
%t A228939 f[x_, y_] := {{y, x* y}, {y, x^2 + y^2}}; x = 1; y = 2; t = {{x, y}};
%t A228939 u = Table[t = Flatten[Map[Apply[f, #] &, t], 1], {18}]; v = Flatten[u];
%t A228939 w = Flatten[Prepend[Table[v[[2 k]], {k, 1, Length[v]/2}], {x, y}]];
%t A228939 Sort[Union[w]]
%Y A228939 Cf. A228853, A228939.
%K A228939 nonn,easy
%O A228939 1,2
%A A228939 _Clark Kimberling_, Sep 08 2013