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

Terms

a(0) =0a(1) =0a(2) =0a(3) =3a(4) =6a(5) =21a(6) =36a(7) =90a(8) =150a(9) =339a(10) =540a(11) =1149a(12) =1806a(13) =3765a(14) =5790a(15) =11880a(16) =18150a(17) =36894a(18) =55980a(19) =113145a(20) =170970a(21) =344541a(22) =519156a(23) =1043190a(24) =1569744a(25) =3149979a(26) =4733670a(27) =9488409a(28) =14250606a(29) =28544205