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Homework Help: Branch cuts

  1. Mar 13, 2008 #1
    We are asked to find a branch where the multi-valued function is analytic in the given domain.

    The function: [itex](4+z^2)^{1/2}[/itex] in the complex plane slit along the imaginary axis from -2i to 2i.

    The principal branch is [itex]\exp(\frac{1}{2} Log(4+z^2))[/itex] and so we want analyticity from -2i to 2i. It seems to me that this is obeyed and the cuts appear when |z|>2i. When |z|>2i we see that the term inside Log become negative, when |z|<2i,the term inside the Log is positive. Does this mean the principal branch is a branch that is analytic in the given domain? I referred to the back of the book and it gave a different branch, namely [itex]z \exp(\frac{1}{2} Log(1+\frac{4}{z^2})})[/itex], what went wrong?

    The function is: [itex](z^4-1)^{1/2}[/itex] in |z|>1.

    The principal branch is [itex]\exp(\frac{1}{2}Log(z^4-1))[/itex] and so we want analyticity when |z|>1. However, we see that this is already the case. Because within the unit circle, z^4 is positive, and subtracting 1 makes z^4-1 negative. We want analyticity when |z|>1 which is the case since z^4 is now positive and larger than 1, making z^4-1 positive when |z|>1. Again, I feel that the principal branch works, however, the back of the book gives a different result [itex]z^2 \exp{\frac{1}{2} Log(1-\frac{1}{z^4})}[/itex]

    The function in d is: [itex](z^3-1)^{1/2}[/itex] in |z|>1.

    I could make the same argument as above, and say the principal branch of [itex]\exp(\frac{1}{2} Log(z^3-1))[/itex] works. However, again, this disagrees with the result in the back of the text: [itex]z\exp(\frac{1}{3} Log(1-\frac{1}{z^3}))[/itex]
     
    Last edited: Mar 13, 2008
  2. jcsd
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