# Prove convergence of the sequence $(z_1+z_2+\cdots + z_n)/n$ of Cesaro means

Prove that if $\lim_{n \to \infty}z_{n}=A$ then: $$\lim_{n \to \infty}\frac{z_{1}+z_{2}+\cdots + z_{n}}{n}=A$$ I was thinking spliting it in: $$(z_{1}+z_{2}+\cdots+z_{N-1})+(z_{N}+z_{N+1}+\cdots+z_{n})$$ where $N$ is value of $n$ for which $|A-z_{n}|<\epsilon$ then taking the limit of this sum devided by $n$ , and noting that the second sum is as close as you wish to $nA$ while the first is as close as you wish to $0$. Not sure if this helps....

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Possibly easier to first show for $A=0$ – Thomas Andrews Oct 5 '12 at 19:19
@Thomas Andrews I edited the question, adding my idea. Could you tell me what you think, please? – Mykolas Oct 5 '12 at 19:29
Changed the title. There is no series here. – Did Oct 5 '12 at 20:54
Maybe this question was asked here before (I did not search), but at least I remember that we had questions such that this can be obtained as a consequence of the results from those questions, see, e.g., If $\sigma_n=\frac{s_1+s_2+\cdots+s_n}{n}$ then $\limsup\sigma_n \leq \limsup s_n$ and limit of quotient of two series. – Martin Sleziak Oct 6 '12 at 9:28

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– Martin Sleziak Oct 27 '12 at 6:53
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## 1 Answer

It seems like Homework problem,hence I'll just give hint: $$\frac{z_1+z_2+\cdots +z_n}{n}-A=\frac {(z_1-A)+(z_2-A)+\cdots +(z_n-A)}{n}$$ Now use the defn of limit that for every $\epsilon > 0$ there exists $N_0 \in \mathbb N$ such that $|z_m-A| < \epsilon \ \forall m \geq N_0$

Also remember triangle inequality : $|a_1+a_2+\cdots +a_n| \leq |a_1| + |a_2| +\cdots +|a_n|$

Can You find proper $a_i$ in terms of say $z_i$'s??

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thankyou for help. It's not homework. Would $a_{i}$ be  $\frac{z_{i}}{n}$, and since for $n \to\infty$all sums go to $0$, it would be proved? @TheJoker – Mykolas Oct 5 '12 at 19:40
and would my first idea be wrong? – Mykolas Oct 5 '12 at 19:44
You can take $a_i=\frac{z_1-A}{n}$. Now use the fact that limit of $z_i$ is $A$. about your approach,it is true,just write rigorously :-).also there is no dofference between mine and your approach as your next step would be same. – TheJoker Oct 5 '12 at 19:51
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