%I #12 May 14 2022 11:20:53
%S 3,312,312112,31211211112,31211211112111111112,
%T 3121121111211111111211111111111111112,
%U 3121121111211111111211111111111111112111111111111111111111111111111112
%N Image of 3 under repeated applications of the morphism 1 -> 11, 2 -> 2, 3 -> 312.
%C a(n) has length 2^n + n.
%D J.-P. Allouche and J. Shallit, Automatic Sequences, Cambridge Univ. Press, 2003, Problem 4, p. 242.
%p b:= proc(n) option remember; `if`(n=0, 3, (l-> subs(3=
%p [3, 1, 2][], subs(1=[1$2][], l))[])([b(n-1)])) end:
%p a:= n-> parse(cat(b(n))):
%p seq(a(n), n=0..6); # _Alois P. Heinz_, May 26 2021
%t FromDigits /@ SubstitutionSystem[{1 -> {1, 1}, 2 -> {2}, 3 -> {3, 1, 2}}, {3}, 6] (* _Jean-François Alcover_, May 14 2022 *)
%K nonn
%O 0,1
%A _N. J. A. Sloane_, Jul 14 2018
%E a(2) corrected by _Georg Fischer_, May 26 2021