Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Residue Theorem

  1. Aug 18, 2012 #1
    1. The problem statement, all variables and given/known data

    Suppose that f is continuous and that there exist constants A,B ≥ 0 and k>1 such that |f(z)|≤A|z|−k for all z such that |z|>B. let CR denote the semicircle given by |z| = R, Re(z) ≥ 0. Prove that limR→∞∫f(z)dz=0

    2. Relevant equations

    3. The attempt at a solution I dont understand what i'm prooving here. I think it has something to do with jordans lemma but I havent a clue! any ideas...please...
  2. jcsd
  3. Aug 18, 2012 #2


    User Avatar
    Homework Helper
    Gold Member

    It is against forum rules to create multiple threads for the same problem.

    Have you learned the estimation lemma yet?
  4. Aug 19, 2012 #3
    Sorry about the multiple thread but I called it jordans lemma then thought, it might have nothing to do with it but I couldnt delete the thread after creating it!

    I havent done the estimation lemma but I looked it up
    Let f : U[itex]\rightarrow[/itex]C be continuous (where U is some subset of C), let [itex]\gamma[/itex] be a path in U, and suppose |f(z)| < M for all z [itex]\in \lambda[/itex]. Let length( [itex]\lambda[/itex])= L. Then

    |[itex]\int[/itex]f(z)dz| [itex]\leq[/itex] ML.
  5. Aug 19, 2012 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    I deleted the other thread. Next time, just hit the report button and ask a mentor to edit or delete the thread for you.
  6. Aug 19, 2012 #5


    User Avatar
    Homework Helper
    Gold Member

    It is probably in your textbook, but just not named. Many texts (c.f. Mathematical Methods for Physicists A Concise Introduction Tai L. Chow p. 255) simply list it as a property of complex integrals and give a short proof of it.

    What is the arclength [itex]L[/itex] of your semicircular contour CR? If [itex]|f(z)| \leq A|z|^{-k}[/itex] (which is what I assume you meant when you wrote |f(z)|≤A|z|−k) and [itex]|z|>B>0[/itex], what is an upper bound for [itex]|f(z)|[/itex] on your semi-circular contour?

    What does that tell you about [itex]\left| \int_{CR} f(z) dz \right|[/itex]?
  7. Aug 19, 2012 #6
    The arc lenght is[itex]\pi r[/itex]

    [itex] |f(z)| \leq A|z|^{-k} [/itex] the semicircle is given by |z| = R so for k>1, if the value of R gets bigger, then the value of [itex] \frac{A}{|z|^{k}} [/itex] gets smaller so as R[itex]\rightarrow\infty[/itex] then [itex] |f(z)| \rightarrow [/itex] 0
    I guess thats what ther're saying but i guess i better try and word it better
  8. Aug 19, 2012 #7


    User Avatar
    Homework Helper
    Gold Member

    Right or, using the estimation lemma with the fact that [itex]|f(z)| \leq A|R|^{-k} [/itex], you have

    [tex]\left| \lim_{R \to \infty} \int_{CR} f(z)dz \right| = \lim_{R \to \infty} \left| \int_{CR} f(z)dz \right| \leq \lim_{R \to \infty} \left( A|R|^{-k} \right) = 0[/tex]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook