Number of permutations of length n which avoid the patterns 231, 4123.

Terms

a(0) =1a(1) =2a(2) =5a(3) =13a(4) =33a(5) =82a(6) =202a(7) =497a(8) =1224a(9) =3017a(10) =7439a(11) =18343a(12) =45228a(13) =111514a(14) =274945a(15) =677894a(16) =1671393a(17) =4120937a(18) =10160465a(19) =25051354a(20) =61765902a(21) =152288233a(22) =375477484a(23) =925766477a(24) =2282543187a(25) =5627772815a(26) =13875674756a(27) =34211464510a(28) =84350802705