a(n) = (7*n^4 + 5*n^2)/12.

Terms

a(0) =1a(1) =11a(2) =51a(3) =156a(4) =375a(5) =771a(6) =1421a(7) =2416a(8) =3861a(9) =5875a(10) =8591a(11) =12156a(12) =16731a(13) =22491a(14) =29625a(15) =38336a(16) =48841a(17) =61371a(18) =76171a(19) =93500a(20) =113631a(21) =136851a(22) =163461a(23) =193776a(24) =228125a(25) =266851a(26) =310311a(27) =358876a(28) =412931a(29) =472875