Number of permutations of length n within distance 5.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =24a(5) =120a(6) =720a(7) =3720a(8) =17304a(9) =76110a(10) =329462a(11) =1441923a(12) =6487445a(13) =29555588a(14) =135025756a(15) =615260976a(16) =2791161792a(17) =12618600768a(18) =57008446080a(19) =257708989200