Number of squarefree quaternary words of length n.

Terms

a(0) =1a(1) =4a(2) =12a(3) =36a(4) =96a(5) =264a(6) =696a(7) =1848a(8) =4848a(9) =12768a(10) =33480a(11) =87936a(12) =230520a(13) =604608a(14) =1585128a(15) =4156392a(16) =10895952a(17) =28566216a(18) =74887056a(19) =196322976a(20) =514662960a(21) =1349208600a(22) =3536962584a(23) =9272217936a(24) =24307198464a(25) =63721617888a(26) =167046745992a(27) =437914664688