Number of permutations of length n which avoid the patterns 1234, 1324, 3421.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =73a(5) =229a(6) =629a(7) =1521a(8) =3304a(9) =6578a(10) =12201a(11) =21353a(12) =35607a(13) =57007a(14) =88153a(15) =132293a(16) =193422a(17) =276388a(18) =387005a(19) =532173a(20) =720005a(21) =959961a(22) =1262989a(23) =1641673a(24) =2110388a(25) =2685462a(26) =3385345a(27) =4230785a(28) =5245011a(29) =6453923