# Cauchy Riemann Equations (basic doubt)

1. Dec 20, 2010

Lets say we have a function of a complex variable z , f(z).

I read that for the function to be differentiable at a point z0 , the CR equations are a necessary condition but not a sufficient condition.

Can someone give me an example where the CR equations hold but the function is not differentiable at that point , thus justifying that the CR equations holding true aren't sufficient test.

I am unable to visualise.

2. Dec 20, 2010

### HallsofIvy

Staff Emeritus
Let g(x, y)= 1 if xy is not 0, 0 if xy= 0 and let f(z)= g(x,y)(1+ i)= g(x, y)+ ig(x,y) where z= x+ iy. Then
$$\frac{g(x,y)}{\partial x}= \frac{\partial g(x,y)}{\partial y}= 0$$
for (x,y)= (0, 0) so the Riemann-Cauchy equations are satisfied there but the function is not even continuous at (0, 0).

3. Dec 20, 2010