Number of quaternary cubefree words of length n.

Terms

a(0) =1a(1) =4a(2) =16a(3) =60a(4) =228a(5) =864a(6) =3264a(7) =12336a(8) =46632a(9) =176208a(10) =665892a(11) =2516412a(12) =9509364a(13) =35935476a(14) =135798588a(15) =513176076a(16) =1939267560a(17) =7328398344