a(n) = (1/24)*(n + 3)*(3*n^3 + 5*n^2 - 6*n + 16).

Terms

a(0) =3a(1) =10a(2) =31a(3) =77a(4) =162a(5) =303a(6) =520a(7) =836a(8) =1277a(9) =1872a(10) =2653a(11) =3655a(12) =4916a(13) =6477a(14) =8382a(15) =10678a(16) =13415a(17) =16646a(18) =20427a(19) =24817a(20) =29878a(21) =35675a(22) =42276a(23) =49752a(24) =58177a(25) =67628a(26) =78185a(27) =89931a(28) =102952a(29) =117337