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Let $\{s_n\}$ be a complex sequence. Let $\sigma_n = (\sum_{i=0}^n s_i)$/$(n+1)$.

Say $s_n → s$.

I have tried and i could only show that $|\sigma_n| →|s|$.

How do i show that $\sigma_n →s$?

I have searched for it and some solutions use "inequality of complex number", but as you know order relation cannot be defined in $\mathbb{C}$.

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    @Brian It's a typo. Dividing by n+1 is what i want!2012-10-05

1 Answers 1

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Hint:

  1. Fix $\varepsilon>0$ and choose $N=N_\varepsilon$ such that $|s_n-s|<\varepsilon$ for $n>N$, and note that $|\sigma_n - s|=\left|\frac{1}{n+1}\sum_0^n (s_k-s)\right|\leq\frac{1}{n+1}\sum_0^n |s_k-s|\tag{1}$
  2. Break up the sum in (1) at $k=N$.

  3. Use the max esitmate on the first terms and the $\varepsilon$-estimate on the tail.

Do you see how to complete the proof?


A sequence $s_n$ for which $\sigma_n$ converges is called Cesàro-summable, the result is that convergence implies Cesàro-summable.

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    Yes, i got it thank you :)2012-10-05