Thomson et al. provide a proof that limn→∞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:
limn→∞lognn=0
Then using this, I can instead prove:
limn→∞n√n=limn→∞explognn=exp0=1
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:
limn→∞n√n=limn→∞explognn
I know such an identity would hold for bounded n but I'm not sure I can use this identity when n→∞.
If I am correct, then would there be any cases where I would be wrong? Specifically, given any sequence xn, can I always assume:
limn→∞xn=limn→∞exp(logxn)
Or are there sequences that invalidate that identity?
(Edited to expand the last question)
given any sequence xn, can I always assume:
limn→∞xn=exp(loglimn→∞xn)=exp(limn→∞logxn)=limn→∞exp(logxn)
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 limn→∞n√n=1.
Answer
Since x↦logx is a continuous function, and since continuous functions respect limits:
limn→∞f(g(n))=f(limn→∞g(n)),
for continuous functions f, (given that limn→∞g(n) exists), your proof is entirely correct. Specifically,
log(limn→∞n√n)=limn→∞lognn,
and hence
limn→∞n√n=exp[log(limn→∞n√n)]=exp(limn→∞lognn)=exp(0)=1.
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