a(n) = n*(n + 1)*(13*n^2 + 13*n - 14)/24.

Terms

a(0) =0a(1) =1a(2) =16a(3) =71a(4) =205a(5) =470a(6) =931a(7) =1666a(8) =2766a(9) =4335a(10) =6490a(11) =9361a(12) =13091a(13) =17836a(14) =23765a(15) =31060a(16) =39916a(17) =50541a(18) =63156a(19) =77995a(20) =95305a(21) =115346a(22) =138391a(23) =164726a(24) =194650a(25) =228475a(26) =266526a(27) =309141a(28) =356671a(29) =409480