a(n) = n*Product_{k=1..n} floor(k^((n-k)/(n-k+1))).

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =20a(5) =48a(6) =112a(7) =864a(8) =2916a(9) =34560a(10) =152064a(11) =1728000a(12) =9360000a(13) =108864000a(14) =1049760000a(15) =10668672000a(16) =172802851200