Number of primitive (period n) periodic palindromes using a maximum of three different symbols.

Terms

a(0) =3a(1) =3a(2) =6a(3) =12a(4) =24a(5) =42a(6) =78a(7) =144a(8) =234a(9) =456a(10) =726a(11) =1392a(12) =2184a(13) =4290a(14) =6528a(15) =12960a(16) =19680a(17) =39078a(18) =59046a(19) =117600a(20) =177060a(21) =353562a(22) =531438a(23) =1061280a(24) =1594296a(25) =3186456a(26) =4782726a(27) =9561552a(28) =14348904a(29) =28690752