a(n) = (3/8)*(n-1)*(n-2)*(27*n^2-137*n+180).

Terms

a(0) =0a(1) =0a(2) =9a(3) =144a(4) =765a(5) =2475a(6) =6120a(7) =12789a(8) =23814a(9) =40770a(10) =65475a(11) =99990a(12) =146619a(13) =207909a(14) =286650a(15) =385875a(16) =508860a(17) =659124a(18) =840429a(19) =1056780a(20) =1312425a(21) =1611855a(22) =1959804a(23) =2361249a(24) =2821410a(25) =3345750a(26) =3939975a(27) =4610034a(28) =5362119a(29) =6202665