How to prove sequence converges quadratically to a root of multiplicity

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Discussion Overview

The discussion revolves around proving that a sequence converges quadratically to a root of multiplicity greater than one for a given function. The focus is on the mathematical formulation of the iteration process and the application of Taylor's series to derive the convergence properties.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant defines a root of multiplicity $m>1$ and presents the iteration formula $x_{k+1}=x_k-mf(x_k)/f'(x_k)$, seeking to prove quadratic convergence under the condition that $f^{(m)}(x_*)≠0$.
  • Another participant requests the original poster to share their working or thoughts to better assist them, indicating a collaborative approach to problem-solving.
  • A third participant suggests that the problem resembles a previous discussion and encourages the original poster to adapt that solution, implying that there may be established methods to tackle similar problems.
  • A later reply expresses disappointment in the original poster's progress, suggesting that the problem may be more challenging than initially thought.

Areas of Agreement / Disagreement

There is no consensus on the proof or the approach to take, as participants express varying levels of confidence and understanding of the problem. Multiple viewpoints and suggestions are presented without resolution.

Contextual Notes

Participants have not provided complete working solutions or assumptions, and the discussion lacks specific mathematical steps that might clarify the convergence proof.

i_a_n
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A function f has a root of multiplicity $m>1$ at the point $ x_*$ if $f(x_*)=f'(x_*)=...=f^{(m-1)}(x_*)=0$. Assume that the iteration$ x_{k+1}=x_k-mf(x_k)/f'(x_k)$ converges to $x_*$. If$ f^{(m)}(x_*)≠0$, prove that this sequence converges quadratically.(We may use the Taylor's series, but I cannot get the result we need to prove.
Expand expand $f(x_k)$ around $x_*$ until $m$-th order derivative term, which has the form
$(x_k - x_*)^m f^{(m)} (x_k) / m!$, and similarly for $ f ' (x_k)$ )
 
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Re: how to prove sequence converges quadratically to a root of multiplicity

In an effort to let our helpers know where you are stuck, can you post your working so far and/or your thoughts on what you should try?
 
Re: how to prove sequence converges quadratically to a root of multiplicity

ianchenmu said:
A function f has a root of multiplicity $m>1$ at the point $ x_*$ if $f(x_*)=f'(x_*)=...=f^{(m-1)}(x_*)=0$. Assume that the iteration$ x_{k+1}=x_k-mf(x_k)/f'(x_k)$ converges to $x_*$. If$ f^{(m)}(x_*)≠0$, prove that this sequence converges quadratically.(We may use the Taylor's series, but I cannot get the result we need to prove.
Expand expand $f(x_k)$ around $x_*$ until $m$-th order derivative term, which has the form
$(x_k - x_*)^m f^{(m)} (x_k) / m!$, and similarly for $ f ' (x_k)$ )


Hey!
This looks a lot like the other thread, where I posted http://www.mathhelpboards.com/f16/optimization-problem-Newtons-method-3447/#post15282.
Heck, you can even copy and paste it, and tweak it a little to be more generalized.
How far can you get?
 
Re: how to prove sequence converges quadratically to a root of multiplicity

Hmm, I thought you would be able to do this.
I guess I was wrong.
Ah well, it wouldn't be the first time I was wrong.
 

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