a(n) = Sum_{d|n} (-1)^(n/d+1)*d!.

Terms

a(0) =1a(1) =1a(2) =7a(3) =21a(4) =121a(5) =715a(6) =5041a(7) =40293a(8) =362887a(9) =3628681a(10) =39916801a(11) =479000895a(12) =6227020801a(13) =87178286161