Number of different positive braids with n crossings of 4 strands.

Terms

a(0) =1a(1) =3a(2) =8a(3) =19a(4) =43a(5) =94a(6) =202a(7) =429a(8) =905a(9) =1901a(10) =3983a(11) =8332a(12) =17413a(13) =36370a(14) =75938a(15) =158519a(16) =330862a(17) =690523a(18) =1441082a(19) =3007369a(20) =6275917a(21) =13096737a(22) =27330418a(23) =57033206a(24) =119016808a(25) =248363751a(26) =518283950a(27) =1081551220a(28) =2256972626a(29) =4709831964