Author Topic: Quiz4 Lec5101 5E  (Read 117 times)

Yuyan Liu

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Quiz4 Lec5101 5E
« on: November 18, 2020, 09:40:37 PM »
Problem: Evaluate the given integral using the technique of Example 10 of Section 2.3:

$$\int_\gamma \frac{dz}{z^3} , $$

where $\gamma$ is any curve in $\{z: Re z \geq 0, z \neq 0 \}$, joining $-i$ to $1 + i$.


Answer:

since f is analytic in all $Re(z) \geq 0, z \neq 0, \int_\gamma \frac{dz}{z^3} = F(1+i) - F(-i)$

$$ F'(z) = f(z) \Rightarrow  F(z) = - \frac{1}{2z^2}$$
$$ F(1+i) = - \frac{1}{2(1+i)^2} = - \frac{1}{4i}$$
$$ F(-i) = - \frac{1}{2(-i)^2} =  \frac{1}{2}$$

Thus, $ \int_\gamma \frac{dz}{z^3} = F(1+i) - F(-i) = - \frac{1}{4i} -  \frac{1}{2} =  \frac{2i-1}{4i}$