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Homework Help: Cauchy-Reimann from first principles

  1. Aug 21, 2012 #1
    1. The problem statement, all variables and given/known data

    Prove from first principles that f(z) = [itex]\overline{z}^2[/itex] is not differentiable at any point z ≠ 0

    2. Relevant equations



    3. The attempt at a solution

    So i guess i Have to show [itex]\stackrel{lim}{h\rightarrow0} \frac{f(z+h)-f(z)}{h}[/itex] is equal to zero right?

    [itex] \frac{\overline{z+h}^2-\overline{z}^2}{h}[/itex] but not sure where to go from here.
    Do i use z=(x+iy) then [itex]\overline{z} = x-iy [/itex] and so [itex]\overline{z}^2[/itex] = [itex]x^2-y^2-i2xy [/itex]
     
  2. jcsd
  3. Aug 21, 2012 #2
    No, you need to prove that it does not exist. Start by formulating "limit does not exist" in the epsilon-delta language.
     
  4. Aug 21, 2012 #3
    You could assume the limit exists (and hence f is differentiable), and calculate it twice using two sequences which converge to zero, and find that you get two different limits. In this case, your assumption becomes false, so f isn't differentiable.

    You do know that the definition of differentiability you have is equivalent to having the limit
    [itex]\lim_{n\rightarrow\infty}\frac{f(z+x_n)-f(z)}{x_n}[/itex]
    exist for every (complex!) sequence [itex]x_n\rightarrow 0[/itex], right?

    In this case, I recommend you work with the definition you have, and see what happens when you approach zero along these two sequences:
    [itex] x_n=1/n[/itex]
    [itex]y_n=i/n[/itex].

    In general, such an approach is usually the best way to show that something -isn't- differentiable. Unless of course you have the Cauchy-Riemann equations :)
     
    Last edited: Aug 21, 2012
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