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

Terms

a(0) =1a(1) =2a(2) =3a(3) =13a(4) =25a(5) =301a(6) =721a(7) =10921a(8) =60481a(9) =740881a(10) =3628801a(11) =106777441a(12) =479001601a(13) =12462690241a(14) =134399865601