Number of permutations of length n which avoid the patterns 1234, 2143, 2431.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =71a(5) =220a(6) =646a(7) =1835a(8) =5095a(9) =13924a(10) =37627a(11) =100859a(12) =268756a(13) =713023a(14) =1885543a(15) =4974068a(16) =13097587a(17) =34440547a(18) =90467764a(19) =237450167a(20) =622857583a(21) =1633072276a(22) =4280258635a(23) =11215502411a(24) =29381846164a(25) =76961231215a(26) =201564237751