Number of permutations of length n which avoid the patterns 1423, 3421.

Terms

a(0) =1a(1) =2a(2) =6a(3) =22a(4) =86a(5) =337a(6) =1299a(7) =4910a(8) =18228a(9) =66640a(10) =240550a(11) =859295a(12) =3043525a(13) =10705182a(14) =37441618a(15) =130351650a(16) =452119862a(17) =1563402141a(18) =5392828631a(19) =18564966510a(20) =63807048144a(21) =219015409556a(22) =750968486726