a(n) = -(n + 1)*(2*n^2 + n - 12)/6.

Terms

a(0) =2a(1) =3a(2) =1a(3) =-6a(4) =-20a(5) =-43a(6) =-77a(7) =-124a(8) =-186a(9) =-265a(10) =-363a(11) =-482a(12) =-624a(13) =-791a(14) =-985a(15) =-1208a(16) =-1462a(17) =-1749a(18) =-2071a(19) =-2430a(20) =-2828a(21) =-3267a(22) =-3749a(23) =-4276a(24) =-4850a(25) =-5473a(26) =-6147a(27) =-6874a(28) =-7656a(29) =-8495