n*(n+1)*(15*n^2-n-8)/12.

Terms

a(0) =0a(1) =1a(2) =25a(3) =124a(4) =380a(5) =905a(6) =1841a(7) =3360a(8) =5664a(9) =8985a(10) =13585a(11) =19756a(12) =27820a(13) =38129a(14) =51065a(15) =67040a(16) =86496a(17) =109905a(18) =137769a(19) =170620a(20) =209020a(21) =253561a(22) =304865a(23) =363584a(24) =430400a(25) =506025a(26) =591201a(27) =686700a(28) =793324a(29) =911905