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 A287404 #4 May 25 2017 17:54:06
%S A287404 3,4,8,11,13,18,19,23,27,28,32,36,38,40,45,48,49,53,56,58,63,64,68,72,
%T A287404 75,76,80,83,85,90,91,95,99,102,103,107,109,113,117,119,121,126,129,
%U A287404 130,134,138,139,143,146,148,153,154,158,162,163,167,171,173,175
%N A287404 Positions of 2 in A287401.
%C A287404 a(n) - a(n-1) is in {1, 2, 3, 4, 5} for n >= 1; also, 3n - a(n) is in {0, 1, 2} for n >= 1. The first 20 numbers 3n - a(n) are 0, 2, 1, 1, 2, 0, 2, 1, 0, 2, 1, 0, 1, 2, 0, 0, 2, 1, 1, 2, with 0 in positions given by A189728.
%H A287404 Clark Kimberling, <a href="/A287404/b287404.txt">Table of n, a(n) for n = 1..10000</a>
%t A287404 s = Nest[Flatten[# /. {0->{0, 1, 2}, 1->{2, 1, 0}, 2->{1, 2, 0}}] &, {0}, 9]; (*A287401*)
%t A287404 Flatten[Position[s, 0]]; (*A189728*)
%t A287404 Flatten[Position[s, 1]]; (*A287403*)
%t A287404 Flatten[Position[s, 2]]; (*A287404*)
%Y A287404 Cf. A287401, A287403, A287404.
%K A287404 nonn,easy
%O A287404 1,1
%A A287404 _Clark Kimberling_, May 25 2017