a(n) = (n^3 - n + 9)/3.

Terms

a(0) =3a(1) =5a(2) =11a(3) =23a(4) =43a(5) =73a(6) =115a(7) =171a(8) =243a(9) =333a(10) =443a(11) =575a(12) =731a(13) =913a(14) =1123a(15) =1363a(16) =1635a(17) =1941a(18) =2283a(19) =2663a(20) =3083a(21) =3545a(22) =4051a(23) =4603a(24) =5203a(25) =5853a(26) =6555a(27) =7311a(28) =8123a(29) =8993