Number of double palindrome structures of length n using exactly two different symbols.

Terms

a(0) =0a(1) =1a(2) =3a(3) =7a(4) =15a(5) =25a(6) =49a(7) =79a(8) =129a(9) =211a(10) =341a(11) =517a(12) =819a(13) =1275a(14) =1863a(15) =2959a(16) =4335a(17) =6703a(18) =9709a(19) =15067a(20) =21367a(21) =33419a(22) =47081a(23) =72949a(24) =102315a(25) =158887a(26) =220881a(27) =342615a(28) =475107a(29) =734815