Number of positive words of length n in the monoid Br_3 of positive braids on 4 strands.

Terms

a(0) =1a(1) =3a(2) =8a(3) =19a(4) =44a(5) =102a(6) =237a(7) =551a(8) =1281a(9) =2978a(10) =6923a(11) =16094a(12) =37414a(13) =86977a(14) =202197a(15) =470051a(16) =1092736a(17) =2540303a(18) =5905488a(19) =13728594a(20) =31915109a(21) =74193627a(22) =172479257a(23) =400965626a(24) =932131991a(25) =2166943978a(26) =5037533578a(27) =11710844769a(28) =27224411129a(29) =63289077427