Number of periodic palindromic structures of length n using a maximum of three different symbols.

Terms

a(0) =1a(1) =1a(2) =2a(3) =2a(4) =5a(5) =5a(6) =12a(7) =14a(8) =33a(9) =41a(10) =90a(11) =122a(12) =259a(13) =365a(14) =756a(15) =1094a(16) =2233a(17) =3281a(18) =6642a(19) =9842a(20) =19813a(21) =29525a(22) =59292a(23) =88574a(24) =177527a(25) =265721a(26) =532170a(27) =797162a(28) =1595443a(29) =2391485