a(n) = Sum_{d|n} (d-1)!.

Terms

a(0) =1a(1) =2a(2) =3a(3) =8a(4) =25a(5) =124a(6) =721a(7) =5048a(8) =40323a(9) =362906a(10) =3628801a(11) =39916930a(12) =479001601a(13) =6227021522a(14) =87178291227