Number of permutations of length n which avoid the patterns 2134, 3142, 3421.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =73a(5) =239a(6) =734a(7) =2133a(8) =5924a(9) =15859a(10) =41202a(11) =104433a(12) =259312a(13) =632815a(14) =1521646a(15) =3612653a(16) =8482796a(17) =19726315a(18) =45481962a(19) =104071145a(20) =236519400a(21) =534249447a(22) =1200095206a(23) =2682257381a(24) =5967445988a(25) =13220446179a(26) =29175578594a(27) =64156073953