Monday, 12 May 2014

real analysis - limntoinftyfrac1nsumnk=1xk=x; given ;limntoinftyxn=x;?



I have a question which is giving me a hard time.



I want to show that lim given that \lim_{n\to \infty} x_n= x.


Answer



Or you could slog through a tedious proof:



Choose \epsilon>0. Let N be such that n\geq N means |x_n-x| < \frac{\epsilon}{2}. Now choose N'\geq N so that n\geq N' means \frac{1}{n} \sum_{k=1}^N |x_n-x| < \frac{\epsilon}{2}.




Then, if n\geq N', we have the estimate:



|\frac{1}{n} \sum_{k=1}^n (x_n-x)| \leq \frac{1}{n} \sum_{k=1}^n |x_n-x| \leq \frac{1}{n} \sum_{k=1}^N |x_n-x| + \frac{1}{n} \sum_{k=N+1}^n |x_n-x| < \frac{\epsilon}{2}+n\frac{1}{n}\frac{\epsilon}{2}= \epsilon



Hence \lim_{n\to\infty} \frac{1}{n} \sum_{k=1}^n (x_n-x) = 0 from which the result follows.


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