Friday, 8 September 2017

Does the limit limntoinftyleft(xn1right)1/n exist?



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




lim





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


No comments:

Post a Comment

real analysis - How to find lim_{hrightarrow 0}frac{sin(ha)}{h}

How to find \lim_{h\rightarrow 0}\frac{\sin(ha)}{h} without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...