Mapping a circle to an ellipse

[joeblow]

How does the function f(z) = z + 1/z take a circle of radius g.t. 1 to an ellipse? How do I think about it geometrically ? (i.e., how should I be able to look at the complex function and tell straight away)

 

[torquil]

How does the function f(z) = z + 1/z take a circle of radius g.t. 1 to an ellipse? How do I think about it geometrically ? (i.e., how should I be able to look at the complex function and tell straight away)

I can't see it by just looking at it, but I did manage to prove it by inserting [tex]z=re^{i\phi}[/tex], then finding x and y-components of the expression f(z) in terms of [tex]\cos(\phi)[/tex] and [tex]\sin(\phi)[/tex]. Then I used the relationship [tex]cos^2 + sin^2 = 1[/tex] to find an expression among x and y, which turned out to be the equation for an ellipse, as long as r>=1.