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 A012212 #16 Aug 27 2018 16:28:41
%S A012212 3,6,6,12,12,12,12,18,18,24,36,48,60,84,102,138,186,246,306,378,486,
%T A012212 630,816,1050,1350,1782,2328,2988,3870,5076,6624,8616,11112,14454,
%U A012212 18882,24630,31944,41442,53988,70422,91614,119046,154896,201834,262824,341688
%N A012212 Number of squarefree palindromes over {0, 1, 2} of length 2n+1.
%H A012212 Giovanni Resta, <a href="/A012212/b012212.txt">Table of n, a(n) for n = 0..55</a>
%t A012212 s = {"1","2","3"}; L=s; a={3}; ext[w_] := Select[#<>w<># & /@ s, StringFreeQ[#, x__ ~~ x__] &]; Do[L = Flatten[ext /@ L, 1]; AppendTo[a, Length@ L], {30}]; a (* _Giovanni Resta_, Aug 26 2018 *)
%Y A012212 Cf. A006156.
%K A012212 nonn,base
%O A012212 0,1
%A A012212 _Jeffrey Shallit_
%E A012212 a(31)-a(45) from _Giovanni Resta_, Aug 26 2018