The number of simple permutations of length n which avoid 1243 and 2431.

Terms

a(0) =1a(1) =2a(2) =0a(3) =2a(4) =4a(5) =10a(6) =21a(7) =44a(8) =89a(9) =178a(10) =352a(11) =692a(12) =1355a(13) =2648a(14) =5171a(15) =10100a(16) =19744a(17) =38646a(18) =75761a(19) =148772a(20) =292653a(21) =576678a(22) =1138240a(23) =2250152a(24) =4454679a(25) =8830640a(26) =17525991a(27) =34820264a(28) =69244864a(29) =137815978