Number of periodic palindromic structures of length n using exactly three different symbols.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =1a(5) =5a(6) =6a(7) =19a(8) =25a(9) =64a(10) =90a(11) =208a(12) =301a(13) =656a(14) =966a(15) =2035a(16) =3025a(17) =6250a(18) =9330a(19) =19035a(20) =28501a(21) =57740a(22) =86526a(23) =174436a(24) =261625a(25) =525994a(26) =788970a(27) =1583119a(28) =2375101a(29) =4760516