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

Terms

a(0) =1a(1) =5a(2) =20a(3) =150a(4) =624a(5) =9600a(6) =36000a(7) =811440a(8) =6572160a(9) =105235200a(10) =442713600a(11) =39437798400a(12) =81430272000