a(n) = n! * Sum_{d|n} mu(d) / (d!)^(n/d).

Terms

a(0) =1a(1) =1a(2) =5a(3) =18a(4) =119a(5) =611a(6) =5039a(7) =37800a(8) =361200a(9) =3515149a(10) =39916799a(11) =471148524a(12) =6227020799a(13) =86497207369