Number of permutations of length n which avoid the patterns 1234, 2431, 3412.

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =71a(5) =213a(6) =561a(7) =1317a(8) =2809a(9) =5536a(10) =10220a(11) =17865a(12) =29823a(13) =47867a(14) =74271a(15) =111897a(16) =164289a(17) =235774a(18) =331570a(19) =457901a(20) =622119a(21) =832833a(22) =1100045a(23) =1435293a(24) =1851801a(25) =2364636a(26) =2990872a(27) =3749761a(28) =4662911a(29) =5754471