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

A019309

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

Terms

    a(0) =1a(1) =4a(2) =12a(3) =48a(4) =180a(5) =720a(6) =2832a(7) =11328a(8) =45132a(9) =180528a(10) =721392a(11) =2885568a(12) =11539440a(13) =46157760a(14) =184619712a(15) =738478848a(16) =2953870260a(17) =11815481040a(18) =47261743632a(19) =189046974528a(20) =756187176720

External references