Monday, May 14, 2018

Prove that the sequence $a_{n+1} =frac{1}{2}left(a_{n}+frac{c}{a_{n}}right)$ is convergent and find its limit



Let $c>0$, $a_{1} = 1$, and
$$a_{n+1} =\frac{1}{2}\left(a_{n}+\frac{c}{a_{n}}\right)$$




I need to:




  1. Show that $a_{n}$ is defined for every $n\geq 1$

  2. Show that this sequence is convergent.

  3. Find its limit.



I proved the first part by showing by induction that this sequence is positive for every $n$. To show that this sequence is convergent I'm thinking of showing that this sequence is a Cauchy series, yet can't figure out how.




For the third part I'm clueless at the moment.


Answer



Hints:




  1. One can prove that if $a_1>0$ and $n \geq 2$ then $a_n \geq \sqrt{c}$.

  2. One can prove that if $a_1>0$ and $n \geq 2$ then $a_{n+1} \leq a_n$. Knowing that and the bound in 1, convergence follows.

  3. Once you know that a recursive sequence is convergent, its limit can only be a fixed point of the recursion mapping, i.e. in your case a solution to $\frac{x}{2}+\frac{c}{2x}=x$.


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