Number of permutations of [n] avoiding {2143, 1423, 1234}.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =21a(5) =74a(6) =259a(7) =905a(8) =3163a(9) =11058a(10) =38664a(11) =135193a(12) =472724a(13) =1652965a(14) =5779907a(15) =20210571a(16) =70670238a(17) =247112450a(18) =864077593a(19) =3021418466a(20) =10564988371a(21) =36942575637a(22) =129177037563a(23) =451693114846