Thursday, 14 November 2019

real analysis - Proof that limnrightarrowinftysqrt[n]n=1



Thomson et al. provide a proof that lim 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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