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What are the possible values $\int_\gamma{\frac{1}{z}}dz$ where $\gamma$ is a path that starts at $z=-i$ and ends $z=2i$ and avoids the origin?

I pick a specific branch cut, for example negative x axis, so $-\pi \leq \theta \leq \pi$. $\mathbb{C} -$branch cut is a simply connected region so integral will be the same over all the paths.

Then I have $\int_\gamma {\frac{1}{z}dz} = \int_\gamma{\frac{d}{dz}(\log(z))dz} = Log(2i) - Log(i) = \ln2 -i\frac{\pi}{2} - \ln(1) - i\frac{\pi}{2} = \ln(2) - i\pi$

Is this correct? I am confused about the 'possible values' in the problem statement

aregak
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1 Answers1

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$\newcommand{\bbx}[1]{\,\bbox[8px,border:1px groove navy]{\displaystyle{#1}}\,} \newcommand{\braces}[1]{\left\lbrace\,{#1}\,\right\rbrace} \newcommand{\bracks}[1]{\left\lbrack\,{#1}\,\right\rbrack} \newcommand{\dd}{\mathrm{d}} \newcommand{\ds}[1]{\displaystyle{#1}} \newcommand{\expo}[1]{\,\mathrm{e}^{#1}\,} \newcommand{\ic}{\mathrm{i}} \newcommand{\mc}[1]{\mathcal{#1}} \newcommand{\mrm}[1]{\mathrm{#1}} \newcommand{\pars}[1]{\left(\,{#1}\,\right)} \newcommand{\partiald}[3][]{\frac{\partial^{#1} #2}{\partial #3^{#1}}} \newcommand{\root}[2][]{\,\sqrt[#1]{\,{#2}\,}\,} \newcommand{\totald}[3][]{\frac{\mathrm{d}^{#1} #2}{\mathrm{d} #3^{#1}}} \newcommand{\verts}[1]{\left\vert\,{#1}\,\right\vert}$

Since we are free to choose $\ds{\gamma}$, a 'nice candidate' is $\ds{\gamma \equiv \braces{{1 \over 2}\,\ic + {3 \over 2}\expo{\ic\theta}\ \left.\vphantom{\large A}\right\vert\ \theta \in \pars{-\,{\pi \over 2},{\pi \over 2}}}}$.

\begin{align} \int_{\gamma}{\dd z \over z} & = \int_{-\pi/2}^{\pi/2} {\pars{3/2}\expo{\ic\theta}\ic\,\dd\theta \over \ic/2 + \pars{3/2}\expo{\ic\theta}} = \left.\ln\pars{{1 \over 2}\,\ic + {3 \over 2}\expo{\ic\theta}} \right\vert_{\ -\pi/2}^{\pi/2} \\[5mm] & = \ln\pars{2i} - \ln\pars{-\ic} = \bracks{\ln\pars{2} + {\pi \over 2}\,\ic} - \bracks{\ln\pars{1} + \pars{-\,{\pi \over 2}\,\ic}} = \bbx{\ln\pars{2} + \pi\ic} \end{align}

Another posibility is to go 'straight' from $\ds{-\ic}$ to $\ds{2\ic}$ where we consider the $\ds{\ln}$-Principal Branch. In such a case the integration is reduced to:

\begin{align} &\lim_{\epsilon \to 0^{+}}\pars{% \int_{-1}^{-\epsilon}{\ic\,\dd y \over \ic y} + \int_{-\pi/2}^{\pi/2}{\epsilon\expo{\ic\theta}\ic\,\dd\theta \over \epsilon\expo{\ic\theta}} + \int_{\epsilon}^{2}{\ic\,\dd y \over \ic y}} \\[5mm] = &\ \lim_{\epsilon \to 0^{+}}\bracks{% \ln\pars{\epsilon} + \pi\ic + \ln\pars{2 \over \epsilon}} = \bbx{\ln\pars{2} + \pi\ic} \end{align}

Felix Marin
  • 89,464