The number of permutations of length n sortable by 3 block interchanges.

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =120a(5) =720a(6) =5040a(7) =32256a(8) =169632a(9) =737364a(10) =2731444a(11) =8875868a(12) =25894376a(13) =69053375a(14) =170694383a(15) =395443223a(16) =866147111a(17) =1806459866a(18) =3608498678a(19) =6937282452a(20) =12887902732a(21) =23216767894a(22) =40675018726a(23) =69480583966a(24) =115975600846a(25) =189528370396a(26) =303753983092