a(n) = n*(n^4-1)/2.

Terms

a(0) =15a(1) =120a(2) =510a(3) =1560a(4) =3885a(5) =8400a(6) =16380a(7) =29520a(8) =49995a(9) =80520a(10) =124410a(11) =185640a(12) =268905a(13) =379680a(14) =524280a(15) =709920a(16) =944775a(17) =1238040a(18) =1599990a(19) =2042040a(20) =2576805a(21) =3218160a(22) =3981300a(23) =4882800a(24) =5940675a(25) =7174440a(26) =8605170a(27) =10255560a(28) =12149985