Number of permutations of length n which avoid the patterns 2143, 3124, 3421.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =71a(5) =220a(6) =630a(7) =1697a(8) =4365a(9) =10842a(10) =26216a(11) =62071a(12) =144519a(13) =331928a(14) =753834a(15) =1695933a(16) =3784913a(17) =8388838a(18) =18481404a(19) =40501523a(20) =88342827a(21) =191889732a(22) =415236446a(23) =895484281a(24) =1925185941a(25) =4127195570a(26) =8824816080a(27) =18824036847a(28) =40063992335a(29) =85094040112