a(n) = n*(n-1)*(n^3 + 21*n^2 - 4*n + 96)/120.

Terms

a(0) =0a(1) =0a(2) =3a(3) =15a(4) =48a(5) =121a(6) =261a(7) =504a(8) =896a(9) =1494a(10) =2367a(11) =3597a(12) =5280a(13) =7527a(14) =10465a(15) =14238a(16) =19008a(17) =24956a(18) =32283a(19) =41211a(20) =51984a(21) =64869a(22) =80157a(23) =98164a(24) =119232a(25) =143730a(26) =172055a(27) =204633a(28) =241920a(29) =284403