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Homework Help: Newton's method

  1. Apr 5, 2010 #1
    We assume that f(x), f'(x) and f''(x) are continuous in [a,b], and that for some [tex]\alpha[/tex] [tex]\in [/tex] (a,b) , we have [tex] f( \alpha )= 0 [/tex] and [tex]f'( \alpha ) \neq 0 [/tex]. We show that if [tex]x_{0}[/tex] is chosen close enough to [tex]\alpha[/tex], the iterates
    [tex]x_{n+1} = x_{n}- \frac{f(x_{n})}{f'(x_{n})}[/tex]
    converge to [tex]\alpha[/tex].

    I tried to use Taylor's expansion for [tex] f( \alpha )[/tex] (centered at [tex]x_{n}[/tex]), and I got to this expression

    [tex]lim_{n \rightarrow \infty} (\alpha -x_{n+1})= lim_{n \rightarrow \infty} - \frac{1}{2} f''(c) \frac{( \alpha - x_{n} )^{2}}{f'(x_{n})}[/tex]

    where [tex] c \in ( \alpha , x_{n} )[/tex]
    and I guess I want the right hand side to be 0 to get to the answer. But I am not sure how to prove this.
     
    Last edited: Apr 6, 2010
  2. jcsd
  3. Apr 5, 2010 #2

    Borek

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    Staff: Mentor

    Your LaTeX is broken so it is hard to guess what you wrote. But what I am missing from your post is - what is the question? Are you trying to prove why Newton's method works?

    And you don't need f'', f' is enough.
     
  4. Apr 5, 2010 #3
    I am sorry, I fixed the latex part. (hopefully it's readable now)

    By using Taylor's expansion, I got an expression, and by letting n go to infinity, if I could make the RHS become 0, I guess I would have the answer to the problem.
    The thing is ,to do that, I am not sure how to use the fact that [tex] x_{0}[/tex] is chosen close enough to [tex]\alpha[/tex].
     
  5. Apr 6, 2010 #4

    Borek

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    Staff: Mentor

    I am still not sure what you trying to do and what for.
     
  6. Apr 6, 2010 #5
    I am just trying to prove that in this case,

    [tex] lim_{n \rightarrow \infty} (\alpha -x_{n+1})=0 [/tex]

    so then the iterate [tex] x_{n+1} [/tex] converges to [tex]\alpha[/tex].
     
    Last edited: Apr 6, 2010
  7. Apr 6, 2010 #6

    Borek

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    Staff: Mentor

    I don't think going through a Taylor expansion is a good idea. Your proof needs f'', but Newton method works even if only f' exists, so your proof will be incomplete.
     
  8. Apr 6, 2010 #7
    In the hypothesis, we have that f ''(x) is continuous in [a,b], so it exists.
    I guess there must be a reason why they wanted that hypothesis in the problem...

    What would be another way to approach this problem?
     
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