The number of 321- and 2143-avoiding permutations of length n.

Terms

a(0) =1a(1) =2a(2) =5a(3) =13a(4) =33a(5) =80a(6) =185a(7) =411a(8) =885a(9) =1862a(10) =3853a(11) =7881a(12) =15993a(13) =32284a(14) =64945a(15) =130359a(16) =261293a(17) =523282a(18) =1047397a(19) =2095781a(20) =4192721a(21) =8386792a(22) =16775145a(23) =33552083a(24) =67106213a(25) =134214750a(26) =268432125a(27) =536867201a(28) =1073737705a(29) =2147479092