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 A287421 #4 May 26 2017 16:43:45
%S A287421 3,5,9,11,15,18,20,24,27,29,33,36,38,42,45,48,50,54,56,60,63,66,68,72,
%T A287421 74,78,81,83,87,90,92,96,99,102,104,108,110,114,117,120,122,126,128,
%U A287421 132,135,138,140,144,146,150,153,155,159,162,164,168,171,173,177
%N A287421 Positions of 2 in A287418.
%C A287421 a(n) - a(n-1) is in {2,3,4} for n >= 1; also, 3n - a(n) is in {0, 1} for n >= 1. The first 20 numbers 3n - a(n) are 0, 1, 0, 1, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, with 1 in positions given by A287420.
%H A287421 Clark Kimberling, <a href="/A287421/b287421.txt">Table of n, a(n) for n = 1..10000</a>
%t A287421 s = Nest[Flatten[# /. {0->{0, 1, 2}, 1->{1, 2, 0}, 2->{1, 0, 2}}] &, {0}, 9]; (*A287418*)
%t A287421 Flatten[Position[s, 0]]; (*A287419*)
%t A287421 Flatten[Position[s, 1]]; (*A287420*)
%t A287421 Flatten[Position[s, 2]]; (*A287421*)
%Y A287421 Cf. A287418, A287419, A287421.
%K A287421 nonn,easy
%O A287421 1,1
%A A287421 _Clark Kimberling_, May 26 2017