Number of permutations of length n which avoid the patterns 3124, 3241, 4321.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =74a(5) =247a(6) =769a(7) =2247a(8) =6238a(9) =16649a(10) =43132a(11) =109257a(12) =272073a(13) =668704a(14) =1626916a(15) =3926643a(16) =9416939a(17) =22468173a(18) =53383884a(19) =126404204a(20) =298455204a(21) =703023125a(22) =1652724170a(23) =3878899500a(24) =9090904341a(25) =21280894049a(26) =49766276975a(27) =116280945813