Number of palindromes of length n using exactly three different symbols.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =6a(5) =6a(6) =36a(7) =36a(8) =150a(9) =150a(10) =540a(11) =540a(12) =1806a(13) =1806a(14) =5796a(15) =5796a(16) =18150a(17) =18150a(18) =55980a(19) =55980a(20) =171006a(21) =171006a(22) =519156a(23) =519156a(24) =1569750a(25) =1569750a(26) =4733820a(27) =4733820a(28) =14250606a(29) =14250606