Mister Exam

# Sum of series factorial(n)*3^n/n^n

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### The solution

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\
\        n
\   n!*3
)  -----
/      n
/      n
/___,
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
False

False
The rate of convergence of the power series
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n = 1          
$$\sum_{n=1}^{\infty} 3^{n} n^{- n} n!$$
Sum(3^n*n^(-n)*factorial(n), (n, 1, oo))