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

Terms

a(0) =1a(1) =4a(2) =12a(3) =60a(4) =240a(5) =1860a(6) =10080a(7) =95760a(8) =766080a(9) =8210160a(10) =79833600a(11) =1100484000a(12) =12454041600a(13) =188172784800