Number of permutations of length n which avoid the patterns 4321 and 2143.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =22a(5) =86a(6) =333a(7) =1235a(8) =4339a(9) =14443a(10) =45770a(11) =138988a(12) =407134a(13) =1157576a(14) =3212157a(15) =8740499a(16) =23416169a(17) =61973483a(18) =162492830a(19) =423077186a(20) =1095978346a(21) =2829227612a(22) =7287399119a(23) =18748151799a(24) =48213813401a(25) =124015241701a(26) =319200003734a(27) =822375826202