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 A309198 #13 Nov 09 2023 11:47:19
%S A309198 1,2,3,4,1,2,3,1,2,3,4,1,2,3,4,1,2,3,1,2,3,4,1,2,3,1,2,3,4,1,2,3,4,1,
%T A309198 2,3,1,2,3,4,1,2,3,4,1,2,3,1,2,3,4,1,2,3,1,2,3,4,1,2,3,4,1,2,3,1,2,3,
%U A309198 4,1,2,3,1,2,3,4,1,2,3,4,1,2,3,1,2,3,4,1,2,3,4,1,2,3,1,2,3,4,1,2
%N A309198 Fixed point of the morphism 1 -> 12, 2 -> 3, 3 -> 4, 4 -> 123.
%H A309198 Rémy Sigrist, <a href="/A309198/b309198.txt">Table of n, a(n) for n = 0..15126</a>
%H A309198 M. Dekking, <a href="https://arxiv.org/abs/1906.08437">Base phi representations and golden mean beta-expansions</a>, arXiv:1906.08437 [math.NT], June 2019.
%t A309198 Nest[Flatten[ReplaceAll[#,{1->{1,2},2->3,3->4,4->{1,2,3}}]]&,{1},10] (* _Paolo Xausa_, Nov 09 2023 *)
%o A309198 (PARI) M = [[1,2], [3], [4], [1,2,3]]; s = [1]; for (k=1, 100, while (k>#s, s=concat(apply(d -> M[d],s))); print1 (s[k] ", ")) \\ _Rémy Sigrist_, Jul 25 2019
%Y A309198 See A308898 for another version.
%K A309198 nonn
%O A309198 0,2
%A A309198 _N. J. A. Sloane_, Jul 24 2019