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

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =120a(5) =540a(6) =1996a(7) =6196a(8) =16732a(9) =40459a(10) =89519a(11) =184185a(12) =356721a(13) =656475a(14) =1156443a(15) =1961563a(16) =3219019a(17) =5130856a(18) =7969228a(19) =12094622a(20) =17977422a(21) =26223198a(22) =37602126a(23) =53082966a(24) =73872046a(25) =101457721a(26) =137660797a(27) =184691431a(28) =245213039a(29) =322413765