a(n) = (4/15)*n*(n - 1)*(n^3 - 9*n^2 + 26*n - 9).

Terms

a(0) =0a(1) =0a(2) =8a(3) =24a(4) =48a(5) =112a(6) =312a(7) =840a(8) =2016a(9) =4320a(10) =8424a(11) =15224a(12) =25872a(13) =41808a(14) =64792a(15) =96936a(16) =140736a(17) =199104a(18) =275400a(19) =373464a(20) =497648a(21) =652848a(22) =844536a(23) =1078792a(24) =1362336a(25) =1702560a(26) =2107560a(27) =2586168a(28) =3147984a(29) =3803408