Number of permutations of length n which avoid the patterns 1234, 2341, 3421.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =75a(5) =248a(6) =735a(7) =1952a(8) =4697a(9) =10378a(10) =21320a(11) =41163a(12) =75363a(13) =131808a(14) =221561a(15) =359742a(16) =566561a(17) =868514a(18) =1299754a(19) =1903649a(20) =2734539a(21) =3859704a(22) =5361555a(23) =7340060a(24) =9915417a(25) =13230986a(26) =17456492a(27) =22791511a(28) =29469251a(29) =37760640