# Is f(z)=x differentiable with respect to z?

1. Dec 5, 2011

### inurface323

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

The only thing given is f(z)=x. However, I am under the assumption that z is a complex variable where z=x+iy. I'm also assuming that x is a real variable.

In this example, I know that f(z)=x is not differentiable with respect to z because it does not satisfy the Cauchy-Riemann Equations but I need to prove this using the limit definition of the derivative.

2. Relevant equations

I used the definition of the limit i.e. limit as h→0 [f(x+h)-f(x)]/h however I'm not quite sure what f(x+h) translates into in this problem.

3. The attempt at a solution

this may be a straight forward question that I'm over thinking but I get lim Δz→0 [Δx/Δz] which does not exist. Is that correct?

This is my first post. I don't know how to enter math notation that is easier to read. Help with that would also be appreciated.

2. Dec 5, 2011

### HallsofIvy

Staff Emeritus
Remember that z is a complex number so the limits are taken in the complex plane. That is two-dimensional so there are an infinite number of ways of taking a limit, not just "from below" and "from above".

The derivative at, say, $z_0= x_0+ iy_0$, is given by $\lim_{h\to 0}\frac{f(z_0+ h)- f(z_0)}{h}$ and h itself is a complex number.

Suppose we take the limit as h approaches 0 along the a line parallel to the real axis. Write f(z)= u(x,y)+ iv(x,y), we have u(x,y)= x and v(x,y)= 0.

$$\lim{h\to 0}\frac{u(x_0+h,y_0)+ iv(x_0+h,y_0)- u(x_0,y_0)- iv(x_0,y_0}{h}= \lim_{h\to 0}\frac{x_0+ h- x_0}{h}= 1$$

Now take the limit as h goes to 0 parallel to the imaginary axis: h is now ih:
$$\lim_{h\to 0}\frac{u(x_0, y_0+ h)+ iv(x_0, y_0+h)- u(x_0,y_0)- iv(x_0,y_0)}{ih}= \lim_{h\to 0}\frac{x_0- x_0}{ih}= 0$$
since those are not the same, the limit as h goes to 0 along different paths is different. The limit itself does not exist.