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

Terms

a(0) =0a(1) =0a(2) =2a(3) =2a(4) =6a(5) =6a(6) =14a(7) =14a(8) =30a(9) =30a(10) =62a(11) =62a(12) =126a(13) =126a(14) =254a(15) =254a(16) =510a(17) =510a(18) =1022a(19) =1022a(20) =2046a(21) =2046a(22) =4094a(23) =4094a(24) =8190a(25) =8190a(26) =16382a(27) =16382a(28) =32766a(29) =32766