a(n) = n^n - n*(n + 1) / 2.

Terms

a(0) =1a(1) =0a(2) =1a(3) =21a(4) =246a(5) =3110a(6) =46635a(7) =823515a(8) =16777180a(9) =387420444a(10) =9999999945a(11) =285311670545