a(n) = (6 + 10*n + 5*n^2 + n^3)/2.

Terms

a(0) =3a(1) =11a(2) =27a(3) =54a(4) =95a(5) =153a(6) =231a(7) =332a(8) =459a(9) =615a(10) =803a(11) =1026a(12) =1287a(13) =1589a(14) =1935a(15) =2328a(16) =2771a(17) =3267a(18) =3819a(19) =4430a(20) =5103a(21) =5841a(22) =6647a(23) =7524a(24) =8475a(25) =9503a(26) =10611a(27) =11802a(28) =13079a(29) =14445