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 A106688 #12 Jul 22 2024 00:49:55
%S A106688 2,4,2,5,6,3,4,3,6,1,4,5,2,4,2,5,6,2,4,1,4,5,2,2,5,6,3,6,3,4,2,5,6,3,
%T A106688 4,3,6,1,4,5,3,4,2,5,6,2,2,5,6,3,6,3,4,3,4,3,6,1,4,5,2,4,1,4,5,2,4,2,
%U A106688 5,6,3,4,3,6,1,4,5,2,4,2,5,6,2,4,1,4,5,2,2,5,6,3,6,2,4,2,5,6,3,4,3,6,1,4,5
%N A106688 Six-symbol substitution: characteristic polynomial: x^6 - 4*x^4 - 3*x^3 - x - 1.
%F A106688 1->{2}, 2->{3, 4}, 3->{2, 4}, 4->{2, 5, 6}, 5->{3, 6}, 6->{1, 4, 5}
%e A106688 [1] -> [2] -> [3,4] -> [2,4,2,5,6] -> [3,4,2,5,6,3,4,3,6,1,4,5] -> ... Lengths of iterates is 1,1,2,5,12,28,... = A034943. - _Michael Somos_
%t A106688 s[1] = {2}; s[2] = {3, 4}; s[3] = {2, 4}; s[4] = {2, 5, 6}; s[5] = {3, 6}; s[6] = {1, 4, 5}; t[a_] := Flatten[s /@ a]; p[0] = {1}; p[1] = t[p[0]]; p[n_] := t[p[n - 1]] aa = p[7]
%o A106688 (PARI) {a(n)=my(A); if(n<1,0, A=[2]; while(length(A)<n||A[1]!=2, A=concat(vector(length(A),k,[[2],[3,4],[2,4],[2,5,6],[3,6],[1,4,5]][A[k]]))); A[n])} /* _Michael Somos_, May 16 2005 */
%K A106688 nonn,uned
%O A106688 0,1
%A A106688 _Roger L. Bagula_, May 13 2005