Number of periodic palindromes using exactly three different symbols.

Terms

a(0) =0a(1) =0a(2) =0a(3) =3a(4) =6a(5) =21a(6) =36a(7) =93a(8) =150a(9) =345a(10) =540a(11) =1173a(12) =1806a(13) =3801a(14) =5796a(15) =11973a(16) =18150a(17) =37065a(18) =55980a(19) =113493a(20) =171006a(21) =345081a(22) =519156a(23) =1044453a(24) =1569750a(25) =3151785a(26) =4733820a(27) =9492213a(28) =14250606a(29) =28550361