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

Terms

a(0) =1a(1) =0a(2) =2a(3) =-6a(4) =2a(5) =0a(6) =2a(7) =-1680a(8) =10082a(9) =0a(10) =2a(11) =-665280a(12) =2a(13) =0a(14) =3632428802a(15) =-36843206400a(16) =2a(17) =0a(18) =2a(19) =-670442572800a(21) =0a(22) =2a(25) =0a(28) =2a(29) =0a(30) =2