Number of "bifix-free" words of length n over a three-letter alphabet.

Terms

a(0) =1a(1) =3a(2) =6a(3) =18a(4) =48a(5) =144a(6) =414a(7) =1242a(8) =3678a(9) =11034a(10) =32958a(11) =98874a(12) =296208a(13) =888624a(14) =2664630a(15) =7993890a(16) =23977992a(17) =71933976a(18) =215790894a(19) =647372682a(20) =1942085088a(21) =5826255264a(22) =17478666918a(23) =52436000754a(24) =157307706054a(25) =471923118162