a(n) = (-n^3 + 9n^2 - 5n + 3)/3.

Terms

a(0) =1a(1) =2a(2) =7a(3) =14a(4) =21a(5) =26a(6) =27a(7) =22a(8) =9a(9) =-14a(10) =-49a(11) =-98a(12) =-163a(13) =-246a(14) =-349a(15) =-474a(16) =-623a(17) =-798a(18) =-1001a(19) =-1234a(20) =-1499a(21) =-1798a(22) =-2133a(23) =-2506a(24) =-2919a(25) =-3374a(26) =-3873a(27) =-4418a(28) =-5011a(29) =-5654