We have \(a_0 = 1\text{,}\) \(a_1 = 1\text{,}\) \(a_2 = 2\text{,}\) \(a_3 = 6\text{,}\) \(a_4 = 24\text{,}\) \(a_5 = 120\text{,}\) and so on. The closed formula is \(a_n = n!\text{.}\)