a(n) = Product_{d divides n} ((d-1)!)^moebius(n/d).

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =24a(5) =60a(6) =720a(7) =840a(8) =20160a(9) =15120a(10) =3628800a(11) =55440a(12) =479001600a(13) =8648640a(14) =1816214400a(15) =259459200a(17) =147026880a(19) =55870214400a(23) =771008958720