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 A287400 #4 May 25 2017 17:53:53
%S A287400 3,4,9,12,13,18,19,24,27,28,33,36,39,40,45,48,49,54,57,58,63,66,67,72,
%T A287400 73,78,81,84,85,90,93,94,99,100,105,108,109,114,117,120,121,126,129,
%U A287400 130,135,138,139,144,147,148,153,154,159,162,163,168,171,174,175
%N A287400 Positions of 2 in A287397.
%C A287400 a(n) - a(n-1) is in {1,3,5} for n >= 1; also, 3n - a(n) is in {0, 2} for n >= 1. The first 20 numbers 3n - a(n) are 0, 2, 0, 0, 2, 0, 2, 0, 0, 2, 0, 0, 0, 2, 0, 0, 2, 0, 0, 2, with 2 in positions given by A287399.
%H A287400 Clark Kimberling, <a href="/A287400/b287400.txt">Table of n, a(n) for n = 1..10000</a>
%t A287400 s = Nest[Flatten[# /. {0->{0, 1, 2}, 1->{2, 1, 0}, 2->{1, 0, 2}}] &, {0}, 9]; (*A287397*)
%t A287400 Flatten[Position[s, 0]]; (*A287398*)
%t A287400 Flatten[Position[s, 1]]; (*A287399*)
%t A287400 Flatten[Position[s, 2]]; (*A287400*)
%Y A287400 Cf. A287397, A287398, A287399.
%K A287400 nonn,easy
%O A287400 1,1
%A A287400 _Clark Kimberling_, May 25 2017