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

Terms

a(0) =0a(1) =1a(2) =1a(3) =6a(4) =1a(5) =180a(6) =1a(7) =5040a(8) =20160a(9) =378000a(10) =1a(11) =59875200a(12) =1a(13) =6235669440a(14) =47221574400a(16) =1a(18) =1a(22) =1