The number of permutations of length n sortable by 3 prefix block transpositions.

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =116a(5) =521a(6) =1877a(7) =5531a(8) =13939a(9) =31156a(10) =63416a(11) =119802a(12) =213006a(13) =360179a(14) =583871a(15) =913061a(16) =1384277a(17) =2042806a(18) =2943994a(19) =4154636a(20) =5754456a(21) =7837677a(22) =10514681a(23) =13913759a(24) =18182951a(25) =23491976a(26) =30034252a(27) =38029006a(28) =47723474a(29) =59395191