# Homework Help: Principal branch of the logarithm

1. Jan 22, 2012

### mxmt

1. The problem statement, all variables and given/known data

Define an analytic branch f(z) of w, such that f(z)=0 for the limit of z->$\infty$

Now what is f(1)?

2. Relevant equations

$w=\frac{z+i}{z-i}$

3. The attempt at a solution

The branch cut of the logarithm is: $(-\infty,0)$
All branches of the logarithm are:
f(z)=Log(z)+iArg(z)=Log(z)+2i$\pi$k

But then f(1)=0, which is wrong.

Last edited: Jan 22, 2012
2. Jan 23, 2012

### mxmt

There was a typo in my first post:

Of course this should be $w=log(\frac{z+i}{z-i})$

Anybody who understands it now?

3. Jan 23, 2012

### mxmt

A guess is that the line segment of z=(i,-i) is mapped onto the normal branch cut of the logarithm (-inf,0). Therefore, f(1)=exp(iPi/2) because this is where i is located in the complex plane.