Number of ternary squareful words of length n.

Terms

a(0) =0a(1) =0a(2) =3a(3) =15a(4) =63a(5) =213a(6) =687a(7) =2127a(8) =6483a(9) =19575a(10) =58905a(11) =176943a(12) =531177a(13) =1593981a(14) =4782513a(15) =14348289a(16) =43045923a(17) =129139119a(18) =387419097a(19) =1162259637a(20) =3486782013a(21) =10460350023a(22) =31381055463a(23) =94143173409