a(n) = (n^3 - 7*n + 12)/6.

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =17a(5) =31a(6) =51a(7) =78a(8) =113a(9) =157a(10) =211a(11) =276a(12) =353a(13) =443a(14) =547a(15) =666a(16) =801a(17) =953a(18) =1123a(19) =1312a(20) =1521a(21) =1751a(22) =2003a(23) =2278a(24) =2577a(25) =2901a(26) =3251a(27) =3628a(28) =4033a(29) =4467