Number of permutations of length n within distance 6.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =24a(5) =120a(6) =720a(7) =5040a(8) =30960a(9) =172200a(10) =899064a(11) =4553166a(12) =22934774a(13) =116914351a(14) =610093513a(15) =3222826972a(16) =17101449940a(17) =90706002192a(18) =479654768640