# Solving a complex integral 1

If $P(z)$ is a polynomial and $C$ denotes the circle $|z-a|=R$ what is the value of $$\int_{C}^{} P(z)d\overline{z}$$ ? The answer in Ahlfors is $-2\pi i R^2 P'(a)$ I don't know if I'm doing it right but I made a substitution $$d\overline{z} = -R^2 \frac{dz}{(z-a)^2}$$

• If $P$ is polynomial, i.e. $P(z) = \sum_{k=0}^n a_k \cdot z^k$ for some $a_k \in \mathbb{C}$, $n \in \mathbb{N}_0$, then $P$ is holomorphic and therefore the integral $\int_C P(z) \, dz$ is equal to 0 for all closed curve $C$. – saz Dec 11 '12 at 16:53
• If the integral was wrt dz then yes it would be trivial but it's wrt d(conjugate of z). – A. Napster Dec 11 '12 at 18:41
• Okay, sorry, I have overlooked that. – saz Dec 11 '12 at 18:50
• I still don't get how to solve this. – d13 Apr 23 '13 at 16:42
• i'm confused about why we get $R^2$ and why use (z-a)^2 ?? – d13 Apr 23 '13 at 17:11

$z=a+Re^{it}\Rightarrow \overline z=a+Re^{-it}\Rightarrow d\overline z=-iRe^{-it}dt$ so $$\int_{C} P(z)d\overline z=-i\int_{0}^{2\pi}P(a+Re^{it})Re^{-it}dt$$
$$=-iR^2\int_{0}^{2\pi}{P(a+Re^{it})\over R^2e^{2it}}Re^{it}dt$$ $$=-iR^2\int_{C}{P(z) dz\over (z-a)^2}=-2i\pi R^2 P'(a)$$ because from cauchy integral formula we get $$f^{(n)} (a)={n!\over 2\pi i}\int_{C}{f(z) dz\over (z-a)^{n+1}}$$