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

Image of a complex function

  1. Nov 5, 2012 #1
    Let [itex]f(z)=z+\frac{1}{z}[/itex], the question is to find the image of this function on [itex]|z|>1[/itex].

    To do so, I tried to find the image of the unit circle which is the interval [-2,2] and so I could not determine our image.

    If also we tried to find the image of f we get
    [itex]u(re^{i\theta})=(r+\frac{1}{r})\cos \theta[/itex]
    [itex]v(re^{i\theta})=(r-\frac{1}{r})\sin \theta[/itex]
    with r>1.
  2. jcsd
  3. Nov 5, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No, [itex]f(re^{iθ})=re^{iθ}+e^{i(-θ)}/r[/itex]
    At r=1 this comes to the same, so the image of |z|=1 is indeed the [-2,2] interval of ℝ. But for |z| > 1 there will be an imaginary component.
    Looks to me like the image is still the whole of ℂ. As you increase r, the image becomes a series of concentric ellipses. A proof might be derivable by showing there are always real solutions to x+iy = z + 1/z.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook