Wednesday, July 10, 2019

real analysis - Proof that $lim_{nrightarrow infty} sqrt[n]{n}=1$



Thomson et al. provide a proof that $\lim_{n\rightarrow \infty} \sqrt[n]{n}=1$ in this book. It has to do with using an inequality that relies on the binomial theorem. I tried to do an alternate proof now that I know (from elsewhere) the following:



\begin{align}
\lim_{n\rightarrow \infty} \frac{ \log n}{n} = 0

\end{align}



Then using this, I can instead prove:
\begin{align}
\lim_{n\rightarrow \infty} \sqrt[n]{n} &= \lim_{n\rightarrow \infty} \exp{\frac{ \log n}{n}} \newline
& = \exp{0} \newline
& = 1
\end{align}



On the one hand, it seems like a valid proof to me. On the other hand, I know I should be careful with infinite sequences. The step I'm most unsure of is:

\begin{align}
\lim_{n\rightarrow \infty} \sqrt[n]{n} = \lim_{n\rightarrow \infty} \exp{\frac{ \log n}{n}}
\end{align}



I know such an identity would hold for bounded $n$ but I'm not sure I can use this identity when $n\rightarrow \infty$.



If I am correct, then would there be any cases where I would be wrong? Specifically, given any sequence $x_n$, can I always assume:
\begin{align}
\lim_{n\rightarrow \infty} x_n = \lim_{n\rightarrow \infty} \exp(\log x_n)
\end{align}

Or are there sequences that invalidate that identity?



(Edited to expand the last question)
given any sequence $x_n$, can I always assume:
\begin{align}
\lim_{n\rightarrow \infty} x_n &= \exp(\log \lim_{n\rightarrow \infty} x_n) \newline
&= \exp(\lim_{n\rightarrow \infty} \log x_n) \newline
&= \lim_{n\rightarrow \infty} \exp( \log x_n)
\end{align}
Or are there sequences that invalidate any of the above identities?




(Edited to repurpose this question).
Please also feel free to add different proofs of $\lim_{n\rightarrow \infty} \sqrt[n]{n}=1$.


Answer



Since $x \mapsto \log x$ is a continuous function, and since continuous functions respect limits:
$$
\lim_{n \to \infty} f(g(n)) = f\left( \lim_{n \to \infty} g(n) \right),
$$
for continuous functions $f$, (given that $\displaystyle\lim_{n \to \infty} g(n)$ exists), your proof is entirely correct. Specifically,
$$

\log \left( \lim_{n \to \infty} \sqrt[n]{n} \right) = \lim_{n \to \infty} \frac{\log n}{n},
$$



and hence



$$
\lim_{n \to \infty} \sqrt[n]{n} = \exp \left[\log \left( \lim_{n \to \infty} \sqrt[n]{n} \right) \right] = \exp\left(\lim_{n \to \infty} \frac{\log n}{n} \right) = \exp(0) = 1.
$$


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