\(a_0 = 0\text{,}\) \(a_1 = 1\text{,}\) \(a_2 = 3\text{,}\) \(a_3 = 6\) \(a_4 = 10\text{.}\) The sequence was described by a closed formula. These are the triangular numbers. A recursive definition is: \(a_n = a_{n-1} + n\) with \(a_0 = 0\text{.}\)
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