a(n) = -1 + 5*n/6 + n^3/6.

Terms

a(0) =-1a(1) =0a(2) =2a(3) =6a(4) =13a(5) =24a(6) =40a(7) =62a(8) =91a(9) =128a(10) =174a(11) =230a(12) =297a(13) =376a(14) =468a(15) =574a(16) =695a(17) =832a(18) =986a(19) =1158a(20) =1349a(21) =1560a(22) =1792a(23) =2046a(24) =2323a(25) =2624a(26) =2950a(27) =3302a(28) =3681a(29) =4088