Sunday, 21 February 2016

real analysis - (yn) bounded sequence and limfrac(xn)(yn)=0 show lim(xn)=0



If (xn) and (yn) are positive real sequences such that (yn) is bounded and lim(xn)(yn)=0, prove that lim(xn)=0



So since (yn) is bounded, there exists a real number M such that for all n natural numbers, (yn)<|M|
So by the limit definition :
lim|(xn)(yn)0|=lim(xn)(yn)<ϵ
So |(xn)|<ϵ(yn)




Now I need to find a natural number N such that for all n>N, |(xn)|<ϵ is this correct which would prove this statement. However I'm having trouble picking something that will work here? Am I on the right track? I have to be very careful also about anything I use to quote the theorem since this is an introduction to real analysis class.



Thanks!


Answer



I would approach this by contradiction:



If the sequence does not converge to 0, then there is a subsequence that is bounded away from 0. So we may fix a>0 such that |xn|>a for infinitely many values of n.



Since the sequence of yn is bounded, say by M, we have $|y_n|a/M$.




This inequality holds for infinitely many n, and we have found a subsequence of (xn/yn) that does not converge to 0.



Does this make sense?


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