# Thread: Limit of complex function

1. ## Limit of complex function

Find $\displaystyle \displaystyle\lim_{z \to 0} \frac{\bar{z}^2}{z}$

Attempt: I thought that this limit might not exist since if you let $\displaystyle z = x + iy$, then when approaching from y-axis, the limit equals $\displaystyle iy$ and when approaching from the x-axis, the limit equals $\displaystyle x$.

However the solution says that limit is actually 0. I'm not sure how to get this result.

Any suggestions?

2. Use $\displaystyle |\bar{z}^2/z|=|z|$.

Regards.

3. Is there a limit law that says that the limit of the modulus is related to the limit of the function in any way?

4. Originally Posted by mukmar
Find $\displaystyle \displaystyle\lim_{z \to 0} \frac{\bar{z}^2}{z}$

Attempt: I thought that this limit might not exist since if you let $\displaystyle z = x + iy$, then when approaching from y-axis, the limit equals $\displaystyle iy$ and when approaching from the x-axis, the limit equals $\displaystyle x$.

However the solution says that limit is actually 0. I'm not sure how to get this result.

Any suggestions?
Setting $\displaystyle z=x + i\ y$ You obtain...

$\displaystyle \displaystyle \frac{\bar{z}^{2}}{z} = \frac{x^{2} - y^{2} -2 i x y}{x+i\ y}= \frac{x^{3} -3 x y^{2}}{x^{2}+y^{2}} + i\ \frac{y^{3} -3 x^{2} y}{x^{2}+y^{2}}$ (1)

Now compute separately the $\displaystyle \lim_{(x,y) \rightarrow (0,0)$ of the real and imaginary part of (1)...

Kind regards

$\displaystyle \chi$ $\displaystyle \sigma$

5. Originally Posted by mukmar
Find $\displaystyle \displaystyle\lim_{z \to 0} \frac{\bar{z}^2}{z}$

Attempt: I thought that this limit might not exist since if you let $\displaystyle z = x + iy$, then when approaching from y-axis, the limit equals $\displaystyle iy$ and when approaching from the x-axis, the limit equals $\displaystyle x$.

However the solution says that limit is actually 0. I'm not sure how to get this result.

Any suggestions?

Another idea: write z in polar form, $\displaystyle z=re^{i\phi}\Longrightarrow z\rightarrow 0\Longleftrightarrow r\rightarrow 0\,,\,\overline{z}=re^{-i\phi}$, so:

$\displaystyle \displaystyle{\frac{\overline{z}^2}{z}=\frac{r}{e^ {i\phi}}\xrightarrow [r\to 0]{} 0}$

Tonio

6. Originally Posted by mukmar
Is there a limit law that says that the limit of the modulus is related to the limit of the function in any way?
Better:

$\displaystyle |\bar{z}^2/z-0|<\epsilon\Leftrightarrow |z-0|<\epsilon\quad (z\neq 0)$

Now, choose $\displaystyle \delta=\epsilon$.

Regards.