Friday, 8 September 2017

Does the limit $lim_{ntoinfty}left(x^n-1right)^{1/n}$ exist?



For $x$ given, what do you think about the following limit?




$$
\lim_{n\to\infty}\left(x^n-1\right)^{1/n}.
$$





What I tried and what are the problems that I am facing:



Let $f(x, n)=\left(x^n-1\right)^{1/n}$. We have:



$$
\log f(x, n)=\dfrac{1}{n}\log\left(x^n-1\right)=\dfrac{1}{n}\log\left(1-x^{-n}\right)+\dfrac{1}{n}\log\left(x^n\right),
$$



first, I do not know if I can apply the log or not? I guess $x$ must be real? and must be positive? what about complex?




Finally,
$$
\lim_{n\to\infty}\left(x^n-1\right)^{1/n}=\log x.
$$


Answer



Here is a down and dirty solution, for large $n$, (and assuming $x>1$, since otherwise how can you take $n$ th root),



$$\frac{1}{2}x^n \leq x^n-1 \leq x^n$$
so




$$\frac{1}{\sqrt[n]{2}}x \leq (x^n-1)^{\frac{1}{n}} \leq x$$
So
$$(x^n-1)^{\frac{1}{n}}
\to x$$


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