Number of dissimilar squarefree quaternary words of length n.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =11a(5) =29a(6) =77a(7) =202a(8) =532a(9) =1395a(10) =3664a(11) =9605a(12) =25192a(13) =66047a(14) =173183a(15) =453998a(16) =1190259a(17) =3120294a(18) =8180124a(19) =21444290a(20) =56217025a(21) =147373441a(22) =386342414a(23) =1012799936a(24) =2655067412a(25) =6960281083a(26) =18246444362a(27) =47833200849a(28) =125395149294a(29) =328724391241