Number of permutations of length n which avoid the patterns 1234, 3412, 3421.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =73a(5) =229a(6) =634a(7) =1562a(8) =3481a(9) =7132a(10) =13622a(11) =24531a(12) =42033a(13) =69031a(14) =109306a(15) =167680a(16) =250193a(17) =364294a(18) =519046a(19) =725345a(20) =996153a(21) =1346745a(22) =1794970a(23) =2361526a(24) =3070249a(25) =3948416a(26) =5027062a(27) =6341311a(28) =7930721a(29) =9839643