# Complex Mapping

1. Apr 18, 2009

### latentcorpse

Let $Q:=\{ z: Re(z)>0, Im(z)>0, |z|<1 \}$ i.e. the quarter disc.

what is the image of Q by the mapping $f(z)=z^2$

by trial and error with various points, my answer is that it takes Q to the semicircle $\{ z: Re(z)>0, |z|<1 \}$

but can't how this explicitly as it's not a mobius transformation with which im used to dealing with.

2. Apr 18, 2009

### n!kofeyn

The best way to find the image of mappings like this is to let $z=re^{i\theta}$ and then look at what your mapping does to this polar representation of all z in your region Q.

3. Apr 19, 2009

### latentcorpse

ok so the modulus will square but thats just 1 again and the argument doubles.

our original angle was from 0 to pi/2
so now we go from 0 to pi
so it will be a semicircle in the upper half plane of radius 1?

4. Apr 19, 2009

### n!kofeyn

Looks good to me.