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

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =61a(5) =146a(6) =302a(7) =561a(8) =961a(9) =1546a(10) =2366a(11) =3477a(12) =4941a(13) =6826a(14) =9206a(15) =12161a(16) =15777a(17) =20146a(18) =25366a(19) =31541a(20) =38781a(21) =47202a(22) =56926a(23) =68081a(24) =80801a(25) =95226a(26) =111502a(27) =129781a(28) =150221a(29) =172986