Number of permutations of length n which avoid the patterns 1234, 1243, 3421.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =73a(5) =223a(6) =587a(7) =1356a(8) =2820a(9) =5395a(10) =9653a(11) =16355a(12) =26487a(13) =41299a(14) =62347a(15) =91538a(16) =131178a(17) =184023a(18) =253333a(19) =342929a(20) =457253a(21) =601431a(22) =781339a(23) =1003672a(24) =1276016a(25) =1606923a(26) =2005989a(27) =2483935a(28) =3052691a(29) =3725483