Number of permutations of length n which avoid the patterns 2431, 3124, 3421.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =73a(5) =240a(6) =754a(7) =2309a(8) =6987a(9) =21036a(10) =63202a(11) =189723a(12) =569311a(13) =1708100a(14) =5124492a(15) =15373695a(16) =46121335a(17) =138364290a(18) =415093192a(19) =1245279935a(20) =3735840201a(21) =11207521038a(22) =33622563592a(23) =100867691301a(24) =302603074477a(25) =907809224054