a(n) = (n^3 - 3n^2 + 14n - 6)/6.

Terms

a(0) =1a(1) =3a(2) =6a(3) =11a(4) =19a(5) =31a(6) =48a(7) =71a(8) =101a(9) =139a(10) =186a(11) =243a(12) =311a(13) =391a(14) =484a(15) =591a(16) =713a(17) =851a(18) =1006a(19) =1179a(20) =1371a(21) =1583a(22) =1816a(23) =2071a(24) =2349a(25) =2651a(26) =2978a(27) =3331a(28) =3711a(29) =4119