Number of permutations of length n which avoid the patterns 4231 and 2143.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =22a(5) =86a(6) =335a(7) =1271a(8) =4680a(9) =16766a(10) =58656a(11) =201106a(12) =677767a(13) =2251011a(14) =7382992a(15) =23955716a(16) =77010180a(17) =245577076a(18) =777648145a(19) =2447486221a(20) =7661760386a(21) =23872087936a(22) =74071120682a(23) =228988898916a(24) =705618033237