# Holomorphic at infinity

1. Apr 11, 2006

### quasar987

Hi, could someone explain what it means for a function to be holomorphic on $\mathbb{C}\cup \{\infty\}$? More precisely, what does it mean for it to be holomorphic at $\infty$. Thx.

2. Apr 11, 2006

### shmoe

f(z) is holomorphic at infinity if f(1/z) is holomorphic at 0. Likewise for singularities.

3. Apr 11, 2006

### quasar987

Thx shmoe!

4. Apr 13, 2006

### quasar987

Why do we never talk about continuity and differentiability at infinity for real functions?

5. Apr 13, 2006

### matt grime

because infinity is not part of the real line and not in terms of analysis a useful point to add in, whereas infinity is a very useful adjunction to the complex plane: poles and zeroes are far more important to a study of complex analysis than real analysis. That's just the way it has worked out, and is a very hand wavy explanation. Sorry.

The point is, trying again, that Cu{infinity} is a very important compact riemann surface. It is useful to add this point and talk about functions which take infinity as an admissible value in complex analytic terms and not for real things.

But, of course, we do talk about such things for the extended real line, but they just aren't as useful, and therefore *you* haven't learnt about them.

(You do make a large implicit generalization from what you know about to what *we* know about).

6. Apr 14, 2006

### benorin

Is this the so-called "one point compactification" of either the complex plane or the real line? Would you also expound upon the significance of the complex characteristics of infinity (as, say, a point in the complex plane)?

TIA,
-Ben

7. Apr 14, 2006

### matt grime

Perhaps I should have said 'it has proved more useful'.

It is just useful to allow infinity as a point. what is the integral round a closed curve of a meromorphic function? the sum of the residues at the simple poles. what are simple poles? places where the function takes the value infinity with multiplicity 1. By treating infinity as a genuine point then we can start talking about things more uniformly.

Any holmorphic function from Cu{infinty} to C is constant which is another way of stating that theorem that any bounded holmorphic function is constant.

Last edited: Apr 14, 2006