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Pyroadept
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Homework Statement
State, with justification, if the Fundamental Theorem of Contour Integration can be applied to the following integrals. Evaluate both integrals.
\begin{eqnarray*}
(i) \hspace{0.2cm} \int_\gamma \frac{1}{z} dz \\
(ii) \hspace{0.2cm} \int_\gamma \overline{z} dz \\
\end{eqnarray*}
where
\begin{eqnarray*}
\gamma(t) = \exp(i2\pi t), -\frac{1}{4} \leq t \leq \frac{1}{3}
\end{eqnarray*}
Homework Equations
Fundamental Theorem of Contour Integration:
\begin{eqnarray*}
\text{Suppose }f : S \rightarrow C\text{ is continuous, }\\
\gamma : \left[a:b\right] \rightarrow S\text{ is a smooth path.}\\
\text{Suppose }F : S \rightarrow C\text{ such that }F'(z) = f(z) \text{ }\forall z \in S.\\
\text{Then } \int_\gamma f dz = F(\gamma(b)) - F(\gamma(a))
\end{eqnarray*}Complex Line Integral Formula:
\begin{eqnarray*}
\text{Let }U \subset C \text{ be an open path-connected set. }\\
\text{Let }f : U \rightarrow C \text{ be a continuous function.} \\
\text{Let }\gamma : \left[a:b\right] \rightarrow S \text{ be a smooth path in U.}\\
\text{Then the complex line integral of f along }\gamma \text{ is:} \\
\int_\gamma f dz = \int_{a}^{b} f(\gamma(t))\gamma'(t)dt, \hspace{0.2cm} \text{where }\gamma'(t) = x'(t) + iy'(t)
\end{eqnarray*}
The Attempt at a Solution
Hi everyone,
Here's my attempt so far:\begin{eqnarray*}
\text{(i) }\hspace{0.2cm} \int_\gamma \frac{1}{z} dz
\end{eqnarray*}
Can't use Fundamental Theorem as 1/z has no antiderivative. Even though we can differentiate Log(z) to get 1/z, this is only defined locally, not generally, as Log(z) is itself based on the complex exponential, which is a periodic function.
Therefore, use the complex line integral formula:
\begin{eqnarray*}
\gamma(t) &=& \exp(i2\pi t) \\
\gamma'(t) &=& i2\pi \exp(i2\pi t) \\
f'(\gamma(t)) &=& \frac{1}{\exp(i2\pi t)} \\
&=& \exp(-i2\pi t)\\
\hspace{0.2cm} \int_\gamma \frac{1}{z} dz &=& \int_{-\frac{1}{4}}^{\frac{1}{3}}\exp(-i2\pi t)i2\pi\exp(i2\pi t)dt \\
&=& \int_{-\frac{1}{4}}^{\frac{1}{3}}i2\pi dt \\
&=&\left[i2\pi t\right]_\frac{-1}{4}^\frac{1}{3} \\
&=& i\frac{7\pi}{6}
\end{eqnarray*}\begin{eqnarray*}
\text{(ii) }\hspace{0.2cm} \int_\gamma \overline{z} dz
\end{eqnarray*}
Again, can't use Fundamental Theorem as z conjugate has no antiderivative. Therefore, use the complex line integral formula:
\begin{eqnarray*}
\gamma(t) &=& \exp(i2\pi t) \\
\gamma'(t) &=& i2\pi \exp(i2\pi t) \\
f(\gamma(t)) &=& \overline{\exp(i2\pi t)} \\
&=& \exp(-i2\pi t)\\
\int_\gamma \overline{z} dz &=& \int_{-\frac{1}{4}}^{\frac{1}{3}}\exp(-i2\pi t)i2\pi\exp(i2\pi t)dt \\
&=& i\frac{7\pi}{6} \\
\end{eqnarray*}
as before.
It seems strange that I get the same answer for both parts. I am reasonably happy that my answer for (ii) is correct. I think I may be wrong about the first one, particularly in my reasoning that it has no antiderivative. Also, seeing as the questions are from a chapter on contour integration, it seems strange that neither one can use the fundamental theorem.
I'd really appreciate if anyone could help point out where I may be going wrong!
Thanks