a(n) = n^3/3 - 7*n/3 + 4.

Terms

a(0) =4a(1) =2a(2) =2a(3) =6a(4) =16a(5) =34a(6) =62a(7) =102a(8) =156a(9) =226a(10) =314a(11) =422a(12) =552a(13) =706a(14) =886a(15) =1094a(16) =1332a(17) =1602a(18) =1906a(19) =2246a(20) =2624a(21) =3042a(22) =3502a(23) =4006a(24) =4556a(25) =5154a(26) =5802a(27) =6502a(28) =7256a(29) =8066