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I am a bit confused on all the different ways to show something is holomorphic, and I am wondering if my thoughts about the following are on the right track.

Show $$g(z)=\frac{z^*}{z^2+1}$$ is not holomorphic (where $z^{*}$ denotes the complex conjugate).

I know that a function is holomorphic iff its derivative with respect to $z^*$ is identically zero, but I am not sure I am taking the derivative with respect tot the conjugate correct.

Would it simply be $\frac{\partial g}{\partial z^{*}}=\frac{(z^2+1)(1)-(z^*)(0)}{(z^2+1)^2}=\frac{1}{z^2+1}$ which is only zero for $z=i$, so in general it isn't holomorphic? Is that correct reasoning or no?

And another example of a non holomorphic, $$h(z)=z((z^{*})^2-z^2)$$

$$=(zz^{*})z^{*}-z^2=|z|^{2}z^*-z^2$$

so would the $z^{*}$ derivative just be $|z|^2$ which is zero only at the origin? I'm just confused on how to actually show this. I think I am also confused on taking the deravtive with respect to the conjugate.

I know I could also trying separating into the form of $U(x,y)+IV(x,y)$ and showing that the CR don't hold, but that seems even more difficult. Looking for any and all help/advice. Thank you

Winther
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PersonaA
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2 Answers2

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I'm not sure if the following answers you. If it doesn't, downvote and I'll delete this.

The function $$h(z)=\frac{z^2+1}z$$ is clearly holomorphic at $\Bbb C\setminus\{0\}$.

If $g$ were holomorphic, then $gh$ would be, but $$|gh(z)|=1$$ so, by Maximum Modulus Principle, $gh$ is constant, which is a contradiction.

ajotatxe
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Yes, you are correct. As you mention a complex function $f$ is holomorphic on some open set $G \subset \mathbb{C}$ iff \begin{align} \frac{\partial f}{\partial \bar z} = 0. \end{align} Note we also have that \begin{align} \frac{\partial}{\partial \bar z} = \frac{1}{2}\left(\frac{\partial}{\partial x} + i\frac{\partial}{\partial y} \right). \end{align} which means \begin{align} \frac{\partial g}{\partial \bar z} =&\ \frac{1}{2}\left(\frac{\partial}{\partial x} + i\frac{\partial}{\partial y} \right) \frac{x-iy}{x^2-y^2+1+i2xy}\\ =&\ \frac{(x^2-y^2+1+i2xy)-(x-iy)(2x+i2y)}{2(x^2-y^2+1+i2xy)^2}+i \frac{-i(x^2-y^2+1+i2xy)-(x-iy)(-2y+i2x)}{2(x^2-y^2+1+i2xy)^2}\\ =& \frac{(z^2+1)}{(z^2+1)^2} = \frac{1}{z^2+1} \end{align}

Jacky Chong
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  • Thanks but is the way I differentiated just treating the conjugate as a diffirent variable allowed too? – PersonaA Sep 22 '16 at 23:00
  • Yes, it's allow and you should do it that way. I have included this calculation to show you that you are correct. Of course, I define my $\partial_{\bar z}$ by using $\frac{1}{2}(\partial_x+i\partial_y)$. – Jacky Chong Sep 22 '16 at 23:34
  • @PersonaA Maybe these would be helpful to you: https://en.wikipedia.org/wiki/Wirtinger_derivatives, http://math.stackexchange.com/questions/314863/what-is-the-intuition-behind-the-wirtinger-derivatives – Chill2Macht Oct 14 '16 at 19:57