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\ n
\ n!*3
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/ n
/ n
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n = 1
$$\sum_{n=1}^{\infty} \frac{3^{n} n!}{n^{n}}$$
Sum((factorial(n)*3^n)/n^n, (n, 1, oo))
The radius of convergence of the power series
Given number: $$\frac{3^{n} n!}{n^{n}}$$ It is a series of species $$a_{n} \left(c x - x_{0}\right)^{d n}$$ - power series. The radius of convergence of a power series can be calculated by the formula: $$R^{d} = \frac{x_{0} + \lim_{n \to \infty} \left|{\frac{a_{n}}{a_{n + 1}}}\right|}{c}$$ In this case $$a_{n} = n^{- n} n!$$ and $$x_{0} = -3$$ , $$d = 1$$ , $$c = 0$$ then $$R = \tilde{\infty} \left(-3 + \lim_{n \to \infty}\left(n^{- n} \left(n + 1\right)^{n + 1} \left|{\frac{n!}{\left(n + 1\right)!}}\right|\right)\right)$$ Let's take the limit we find