Wednesday, 12 July 2017

real analysis - liman=0 if left|fracan+1anright|toL<1

Suppose that an0, for every n and that L=lim|an+1an| exists. Show that if L < 1, then liman=0.




What I did so far:



If L < 1 and L=lim|an+1an|, there exists n0 such that for nn0, 0<|an+1|<|an|. That means that the sequence (|an|)nn0 is decreasing.



Consider the set S={|a0|,...,|an0}. S is finite. Let β=max0in0|ai|. Thus, (|an|) is limited (because, for every n, 0|an|β).



Now I have that (|an|) is limited and, throwing away a finite number of terms (the n0 firsts) I can assume that it is decreasing. So I know that $(|a_n|) converges.



How can I prove that it converges to 0?




I also know that if I prove that lim|an|=0, then I have that liman=0.

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