a(n) = n*(n + 1)*(n^2 + 17*n + 54)/24.

Terms

a(0) =0a(1) =6a(2) =23a(3) =57a(4) =115a(5) =205a(6) =336a(7) =518a(8) =762a(9) =1080a(10) =1485a(11) =1991a(12) =2613a(13) =3367a(14) =4270a(15) =5340a(16) =6596a(17) =8058a(18) =9747a(19) =11685a(20) =13895a(21) =16401a(22) =19228a(23) =22402a(24) =25950a(25) =29900a(26) =34281a(27) =39123a(28) =44457a(29) =50315