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

Terms

a(0) =2a(1) =11a(2) =29a(3) =59a(4) =104a(5) =167a(6) =251a(7) =359a(8) =494a(9) =659a(10) =857a(11) =1091a(12) =1364a(13) =1679a(14) =2039a(15) =2447a(16) =2906a(17) =3419a(18) =3989a(19) =4619a(20) =5312a(21) =6071a(22) =6899a(23) =7799a(24) =8774a(25) =9827a(26) =10961a(27) =12179a(28) =13484a(29) =14879