The number of permutations of length n sortable by 2 block transpositions.

Terms

a(0) =1a(1) =2a(2) =6a(3) =23a(4) =89a(5) =295a(6) =827a(7) =2017a(8) =4405a(9) =8812a(10) =16424a(11) =28887a(12) =48413a(13) =77897a(14) =121045a(15) =182513a(16) =268057a(17) =384694a(18) =540874a(19) =746663a(20) =1013937a(21) =1356587a(22) =1790735a(23) =2334961a(24) =3010541a(25) =3841696a(26) =4855852a(27) =6083911a(28) =7560533a(29) =9324429