The exponential factorial or expofactorial is an exponential version of the factorial, recursively defined as \(a_0 = 1\) and \(a_n = n^{a_{n - 1}}\). For example, \(a_6 = 6^{5^{4^{3^{2^1}}}}\). The first few \(a_n\) for \(n = 0, 1, 2, 3, \ldots\) are 1, 1, 2, 9, 262144, ... (OEIS A049384). The next number, 5262144, has 183231 digits and starts with 620606987866087447074832055728467... Exponential factorial of 6 is approximately \(10^{4.829261036 \cdot 10^{183230}}\) and starts with 110356022591769663217914533447534.... In Hyperfactorial array notation, expofactorial is equal to n!1.