# Derivative of a complex function in terms of real and imaginary parts.

1. Jan 4, 2012

### gauss mouse

Hi, I wonder if anyone knows when (maybe always?) it is true that, where

$z=x+iy \text{ and } f : \mathbb{C} \to \mathbb{C} \text{ is expressed as } f=u+iv, \text{ that } f'(z)=\frac{\partial u}{\partial x}+i\frac{\partial v}{\partial x}?$
I'm pretty sure that this is true for $f=exp.$

I should be able to find this but searching google for mathematics is a nightmare.

2. Jan 4, 2012

### quZz

Hi,
if $f(z)$ is analytical function, you can take derivative in any direction on complex plane of $z$, e.g. take it along real axis $dz = dx$.

3. Jan 4, 2012

### micromass

Staff Emeritus
This is indeed true. But remember that a complex differentiable function must satisfy Cauchy-Riemann equations!!

I suggest looking at Theorem I.5.3 of "Complex Analysis" by Freitag and Busam. A free preview is available at google books.

4. Jan 5, 2012

### gauss mouse

Thanks for your help. I should have specified that f be analytic.

Say I wanted to take the derivative in another direction, not along the real or imaginary axis, how would I do that?

Last edited: Jan 5, 2012
5. Jan 5, 2012

### gauss mouse

Thanks for your help. That looks from the preview like a nice book. I'll see if I can find it in the college library.

6. Jan 5, 2012

### micromass

Staff Emeritus
This isn't really a question about real analysis, but rather about multivariable limits.

Say we have the unit vector $(a,b)$. To find the derivative in that direction, we can do this by

$$a\frac{\partial f}{\partial x}+b\frac{\partial f}{\partial y}$$

This can be done if the function is differentiable at the point.

7. Jan 5, 2012

### quZz

Well, you'll get the same answer beacause f(z) is analytic, it follows from definition. You can use Cauchy-Riemann equations to get different forms of df/dz.

8. Jan 5, 2012

### lavinia

A complex differentiable function may be thought of as a real differentiable function whose Jacobian is multiplication by a complex number. As a linear transformation of R^2 a complex number is a rotation followed by a dilation. This gives you the relations between the partial derivatives of f.