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Homework Help: Complex limit

  1. Feb 15, 2010 #1
    1. The problem statement, all variables and given/known data

    whats the [tex] \lim_{z\rightarrow 1+3i} e^{-z} [/tex]

    2. Relevant equations

    do i simply substitute?

    3. The attempt at a solution im out the door on this
  2. jcsd
  3. Feb 15, 2010 #2
    It depends on how you are asked to do it.
    Do you need to prove this is the limit using delta-epsilon definition?

    Anyway the first step would be to evaluate it intuitivley- as you said, substitute.

    Then if you are asked to prove, you will need to use the definition of the limit with your result.
  4. Feb 15, 2010 #3
    well im only asked to evaluate, but the question carries 3 marks , could substitution carry all those marks? How would the epislon delta proof proceed?
  5. Feb 15, 2010 #4
    Once you know:

    [tex]L=e^{-1+3i}[/tex] (whatever number it is)

    You will have to plug it into the definition

    For each [tex]\epsilon[/tex]>0 there exists such [tex]\delta[/tex]>0 so for any z which satisfies [tex]|z+1-3i|<\delta[/tex] it's true that [tex]|e^{z}-e^{-1+3i}|<\epsilon[/tex].

    let z=x+iy, so:


    I used the fact that [tex]|e^{i\theta}|=1[/tex] for any real theta.

    Now y is a real number, therefore cos(y-3) and sin(y-3) are bounded by 1. So:

    [tex](*) \leq \frac{1}{e}\sqrt{(e^{x+1}-1)^{2}+e^{2(x+1)}} \leq \frac{1}{e}\sqrt{2e^{2(x+1)}}=\frac{\sqrt{2}}{e}e^{x+1}=(*)[/tex]

    Notice that:

    [tex]|x+1| \leq \sqrt{(x+1)^{2}+(y-3)^{2}} = |z+1-3i| < \delta [/tex]

    Therefore since delta is positive:
    [tex]x+1< \delta[/tex]

    The exponent is monotonously increasing, so

    [tex](*) < \frac{\sqrt{2}}{e}e^{\delta}[/tex]

    Now the last expression is as small as we want it to be (except for being zero, but the limit definition doesn't require it), so for every choise of epsilon, we can choose such small delta, for which

    [tex]|e^{z}-e^{-1+3i}|< \frac{\sqrt{2}}{e}e^{\delta}< \epsilon[/tex]

    Therefore the limit is really L.
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