Number of permutations according to distance.

Terms

a(0) =0a(1) =1a(2) =2a(3) =4a(4) =10a(5) =24a(6) =55a(7) =128a(8) =300a(9) =700a(10) =1632a(11) =3809a(12) =8890a(13) =20744a(14) =48406a(15) =112960a(16) =263599a(17) =615120a(18) =1435416a(19) =3349624a(20) =7816528a(21) =18240289a(22) =42564706a(23) =99327052a(24) =231785058a(25) =540883000a(26) =1262179815a(27) =2945365040a(28) =6873169028a(29) =16038912628