a(n) = (1/3)*n^3 - n^2 - (1/3)*n - 1.

Terms

a(0) =-1a(1) =-2a(2) =-3a(3) =-2a(4) =3a(5) =14a(6) =33a(7) =62a(8) =103a(9) =158a(10) =229a(11) =318a(12) =427a(13) =558a(14) =713a(15) =894a(16) =1103a(17) =1342a(18) =1613a(19) =1918a(20) =2259a(21) =2638a(22) =3057a(23) =3518a(24) =4023a(25) =4574a(26) =5173a(27) =5822a(28) =6523a(29) =7278