a(n) = C(n,1) + C(n,2) + C(n,3), or n*(n^2 + 5)/6.

Terms

a(0) =0a(1) =1a(2) =3a(3) =7a(4) =14a(5) =25a(6) =41a(7) =63a(8) =92a(9) =129a(10) =175a(11) =231a(12) =298a(13) =377a(14) =469a(15) =575a(16) =696a(17) =833a(18) =987a(19) =1159a(20) =1350a(21) =1561a(22) =1793a(23) =2047a(24) =2324a(25) =2625a(26) =2951a(27) =3303a(28) =3682a(29) =4089