Wednesday, July 10, 2019

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



Thomson et al. provide a proof that limnnn=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:



limnlognn=0



Then using this, I can instead prove:
limnnn=limnexplognn=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:

limnnn=limnexplognn



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:
limnxn=limnexp(logxn)



Or are there sequences that invalidate that identity?



(Edited to expand the last question)
given any sequence xn, can I always assume:
limnxn=exp(loglimnxn)=exp(limnlogxn)=limnexp(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 limnnn=1.


Answer



Since xlogx is a continuous function, and since continuous functions respect limits:
limnf(g(n))=f(limng(n)),


for continuous functions f, (given that limng(n) exists), your proof is entirely correct. Specifically,
log(limnnn)=limnlognn,



and hence



limnnn=exp[log(limnnn)]=exp(limnlognn)=exp(0)=1.


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