Number of quinternary cubefree words of length n.

Terms

a(0) =1a(1) =5a(2) =25a(3) =120a(4) =580a(5) =2800a(6) =13500a(7) =65100a(8) =313940a(9) =1513820a(10) =7299760a(11) =35200020a(12) =169736780a(13) =818482260a(14) =3946776920a(15) =19031623000