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

Terms

a(0) =1a(1) =0a(2) =2a(3) =-3a(4) =2a(5) =5a(6) =2a(7) =-140a(8) =282a(9) =819a(10) =2a(11) =-20482a(12) =2a(13) =133419a(14) =1527528a(15) =-4661085a(16) =2a(17) =-153296429a(18) =2a(19) =1402482796a(20) =36278688162a(21) =13748957859a(22) =2a(28) =2