Number of permutations of length n which avoid the patterns 1234, 2143, 3421.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =69a(5) =181a(6) =375a(7) =651a(8) =1009a(9) =1449a(10) =1971a(11) =2575a(12) =3261a(13) =4029a(14) =4879a(15) =5811a(16) =6825a(17) =7921a(18) =9099a(19) =10359a(20) =11701a(21) =13125a(22) =14631a(23) =16219a(24) =17889a(25) =19641a(26) =21475a(27) =23391a(28) =25389a(29) =27469