Number of ternary words of length n that are not "bifix-free".

Terms

a(0) =0a(1) =0a(2) =3a(3) =9a(4) =33a(5) =99a(6) =315a(7) =945a(8) =2883a(9) =8649a(10) =26091a(11) =78273a(12) =235233a(13) =705699a(14) =2118339a(15) =6355017a(16) =19068729a(17) =57206187a(18) =171629595a(19) =514888785a(20) =1544699313a(21) =4634097939a(22) =13902392691a(23) =41707178073a(24) =125121830427a(25) =375365491281