a(n)=(n^4-n^3-n^2-n)/2.

Terms

a(0) =0a(1) =-1a(2) =1a(3) =21a(4) =86a(5) =235a(6) =519a(7) =1001a(8) =1756a(9) =2871a(10) =4445a(11) =6589a(12) =9426a(13) =13091a(14) =17731a(15) =23505a(16) =30584a(17) =39151a(18) =49401a(19) =61541a(20) =75790a(21) =92379a(22) =111551a(23) =133561a(24) =158676a(25) =187175a(26) =219349a(27) =255501a(28) =295946a(29) =341011