A012212 - OEIS

%I #16 Aug 27 2018 16:28:41

%S 3,6,6,12,12,12,12,18,18,24,36,48,60,84,102,138,186,246,306,378,486,

%T 630,816,1050,1350,1782,2328,2988,3870,5076,6624,8616,11112,14454,

%U 18882,24630,31944,41442,53988,70422,91614,119046,154896,201834,262824,341688

%N Number of squarefree palindromes over {0, 1, 2} of length 2n+1.

%H Giovanni Resta, <a href="/A012212/b012212.txt">Table of n, a(n) for n = 0..55</a>

%t 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 Cf. A006156.

%K nonn,base

%O 0,1

%A _Jeffrey Shallit_

%E a(31)-a(45) from _Giovanni Resta_, Aug 26 2018