Integral of $\int ze^{2z}dz$ on unit circle.

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So I need to compute

$$\int_{\vert z \vert = 1} ze^{2z}dz.$$

Attempt:

I used integration by parts to get

$$\frac{ze^{2z}}{2}\bigg\vert_0^{2\pi} - \int_0^{2\pi} \frac{1}{2}e^{2z}dz$$

which is

$$\pi e^{4\pi} - \bigg(\frac{1}{4}e^{2z} \bigg) \bigg\vert_0^{2\pi}$$

something seems off however, am I on the right track? Namely, are my bounds correct?

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There are 2 best solutions below

0
On

If you wish to neglect application of Cauchy's Integral Theorem, and instead evaluate the integral directly, then we may proceed as follows.

On the unit circle, we note that $|z|=1$ and we may parameterize $z$ as $z=e^{i\phi}$, $\phi\in [0,2\pi]$. Therefore, we have

$$\begin{align} \oint_{|z|=1}ze^{2z}\,dz&=i\int_0^{2\pi}e^{i2\phi}e^{2e^{i\phi}}\,d\phi\\\\ &=\left.\left(\frac14e^{2e^{i\phi}}(2e^{i\phi}-1)\right)\right|_0^{2\pi}\\\\ &=0 \end{align}$$

as expected!

0
On

To add yet another perspective on this, we can compute this by explicitly finding an anti derivative for $f(z)=ze^{2z}$. One antiderivative is $F(z)=\frac14\left(2z-1\right)\mathrm{e}^{2z}$.

The contour integral of $f$ along a contour $\gamma$ beginning at $z_0$ and ending at $z_1$ is: $$\oint_\gamma z e^{2z}dz=F(z_1)-F(z_0)$$ Since in our case $\gamma$ is closed, $z_0$ and $z_1$ will be the same (say $z_0=z_1=0$) and the contour integral is zero.