a(n) = Sum_{p|n, p prime} (n-1)!/(n/p-1)!.

Terms

a(0) =0a(1) =1a(2) =2a(3) =6a(4) =24a(5) =180a(6) =720a(7) =840a(8) =20160a(9) =378000a(10) =3628800a(11) =6985440a(12) =479001600a(13) =6235669440a(14) =47221574400a(15) =259459200