# Branch cuts

1. Sep 28, 2008

### hoffmann

I have a question with regards to branch cuts:

Say I have a function f(z) = log(z2 - 1). Why is a simple branch cut connecting z = -1 and z = +1 not sufficient to define an analytic function? On the other hand, why is it sufficient for the function f(z) = (z^2 - 1)^(1/2) ?

This is in the complex plane where z = a + ib.

2. Sep 28, 2008

### olgranpappy

perhaps the point at infinity?

3. Sep 28, 2008

### hoffmann

^^ what do you mean by this?

4. Sep 29, 2008

### olgranpappy

$$z=1+r_1e^{i\theta_1}$$
$$z=-1+r_2e^{i\theta_2}$$
$$\log(z^2-1)=\log(z-1)+\log(z+1)=\log(r_1r_2)+i(\theta_1+\theta_2)$$