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 A308831 #20 Jun 06 2020 17:01:51 %S A308831 0,1,2,0,0,2,2,1,1,0,0,0,1,0,1,1,1,2,1,0,2,0,2,1,2,2,2,0,1,0,0,1,1,0, %T A308831 1,0,2,1,0,0,2,0,0,0,0,2,1,1,1,1,0,2,2,0,0,1,2,1,1,2,0,1,1,2,2,0,2,0, %U A308831 1,2,2,1,2,1,2,0,2,2,2,2,1,0,1,2 %N A308831 Start with generation 0, which is the empty sequence. For generation N>=1, extend the existing sequence into a non-cyclic ternary de Bruijn sequence of order N. If more than one extension is possible, choose the lexicographically earliest. %C A308831 If using a binary alphabet instead, it would not be possible to extend the sequence infinitely as a de Bruijn sequence (order 3 needs an extra term: 01100010111). - _A. D. Skovgaard_, Apr 19 2020 %H A308831 A. D. Skovgaard, <a href="/A308831/b308831.txt">Table of n, a(n) for n = 0..246</a> %H A308831 A. D. Skovgaard, <a href="/A308831/a308831.py.txt">Python program to generate the sequence, with explanatory comments</a> %H A308831 A. D. Skovgaard, <a href="/A308831/a308831.txt">a(n) for n = 0..246 in compressed notation</a> %e A308831 Generation 1: %e A308831 [012] (All ternary sequences of length 1 now appear. With 3! = 6 solutions, the lexicographically earliest is chosen.) %e A308831 Generation 2: %e A308831 [0120022110] (The sequence is extended from the previous generation, now including all ternary sequences of length 2.) %e A308831 The process continues. %Y A308831 Cf. A080679 (binary equivalent), A166315, A169676. %K A308831 nonn %O A308831 0,3 %A A308831 _A. D. Skovgaard_, Jun 27 2019