# Property of the index of a function

1. Jul 12, 2012

### Jim Kata

Hi, I'm working through a paper and I am quite stupid so some things that maybe obvious are not obvious to me. Say you have some have some complex analytic function that is defined on some simply closed curve, and the index of this function defined on this curve is zero,

$$\int_C \frac{f'(z)}{f(z)}dz =\Delta Arg(f(z)) = 0$$ Then there exists for a specific branch of the logarithm a g(z) such that exp(g(z))=f(z). My question is why do you need the index of f(z) to be zero for this curve for g(z) to be unique on the curve for a specific branch.

2. Jul 12, 2012

### Simon Bridge

Um. Because that's how the math works out.
It sounds a bit like asking if x-a=0 then why does x have to be equal to a to make the relation true?

... what happens when the index is not zero?

3. Jul 12, 2012

### Jim Kata

I'll rephrase the question. Let $$f(z)=R(z)exp(i\theta(z))$$ . Why is it when

$$ind_C(f(z))=0$$ then $$\alpha < \theta (z) < \alpha +2\pi$$ for $$\alpha \in \mathbb{R}$$. When $$z \in C$$

4. Jul 13, 2012

### Simon Bridge

Same answer. That's how the math works out.
Can you think of some way that it shouldn't work out that way?

It can help to understand these things by considering the converse:
What does it mean when indC(f(z)) ≠ 0 ?

What's the paper you are working through?