\[
\newcommand{\Arg}{\mathsf{Arg}}
\newcommand{\C}{\mathbb{C}}
\newcommand{\R}{\mathbb{R}}
\newcommand{\N}{\mathbb{N}}
\newcommand{\Z}{\mathbb{Z}}
\newcommand{\Im}{\mathsf{Im}}
\newcommand{\intd}{\,\mathsf{d}}
\newcommand{\Re}{\mathsf{Re}}
\newcommand{\ball}{\mathsf{B}}
\newcommand{\wind}{\mathsf{wind}}
\newcommand{\Log}{\mathsf{Log}}
\]
Week 5 Worksheet
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Contour Integration
- Determine the value of
\[
\int\limits_\gamma x - y + ix^2 \intd z
\]
where $z = x+iy$ and $\gamma$ is each of the following.
- The straight line joining $0$ to $1+i$.
- The imaginary axis from 0 to 1.
- The line parallel to the real axis from $i$ to $1+i$.
- Calculate from the definition the following contour integrals.
- $\displaystyle\int\limits_{\gamma_1} f$ where $f(z) = \dfrac{1}{z-2}$ and $\gamma_1(t) = 2+2e^{it}$ on $[0,2\pi]$
- $\displaystyle\int\limits_{\gamma_2} f$ where $f(z) = \dfrac{1}{(z-i)^3}$ and $\gamma_2(t) = i+e^{-it}$ on $[0,\pi/2]$
- Let $\gamma$ denote the circular path with centre $1$ and radius $1$, described once anticlockwise and starting at the point $2$. Let $f(z)=|z|^2$. Write down a formula for $\gamma$ and calculate the contour integral of $f$ over $\gamma$.
Antiderivatives
- For each of the following functions find an antiderivative and calculate the integral along any smooth path from $0$ to $i$.
- $f(z) = z^2 \sin(z)$ on $\C$
- $f(z) = z \exp(iz)$ on $\C$
- Put $f(z) = |z|^2$.
- Calculate the contour integral of $f$ over the contour that goes vertically from $0$ to $i$ then horizontally from $i$ to $1+i$.
- Calculate the contour integral of $f$ over the contour that goes horizontally from $0$ to $1$ then vertically from $1$ to $1+i$.
- What does this tell you about possibility of the existence of an antiderivative for $f(z)=|z|^2$?
- The \define{reverse} of a path $\gamma : [a,b] \to \C$ is the path $\tilde{\gamma} : [a,b] \to \C$ defined by $\tilde{\gamma}(t) = \gamma(a + b - t)$.
Let $\gamma$ denote the semi-circular path along a circle with centre $0$ and radius $3$ that starts at $3$ passes through $3i$ and ends at $-3$.
- Write down a formula for $\gamma$.
- Let $f(z)=1/z^2$ on the domain $\C \setminus \{0\}$. Calculate the contour integral of $f$ over $\gamma$.
- Write down a formula for $\tilde{\gamma}$ and calculate the contour integral of $f$ over $\tilde{\gamma}$.
- Verify in this case that $\displaystyle\int\limits_{\tilde{\gamma}} f= - \displaystyle\int\limits_\gamma f$.
- Let $f, g : D \to \C$ be holomorphic. Let $\gamma$ be a smooth path in $D$ starting at $z_0$ and ending at $z_1$. Prove that
\[
\int\limits_\gamma f g' = f(z_1) g(z_1)- f(z_0) g(z_0) - \int\limits_\gamma f' g.
\]
which is the complex analogue of the integration by parts formula.
The Principal Logarithm
- We introduced the principal logarithm via the inversion $\exp(\Log(z)) = z$ for all $z \ne 0$. Here is the other side of that composition.
- What is the range $\{ \Log(z) : z \ne 0 \}$ of the principal logarithm?
- Verify that $\Log(\exp(z)) = z$ for all $z$ in the range of the principal logarithm.
- Give an example of complex numbers $a,b$ for which $\Log(ab) \ne \Log(a) + \Log(b)$.
- Prove that for any two complex numbers $a$ and $b$ there is $n \in \Z$ such that
\[
\Log(ab) = \Log(a) + \Log(b) + 2 \pi i n
\]
holds.
- Calculate $\Log(1+i)$ and $\Log(-1-i)$.
- Given complex numbers $a,b$ with $a \ne 0$ we define $a^b = \exp(b \, \Log(a))$.
- Calculate $i^i$.
- Verify that $a^{b+c} = a^b a^c$ for all $a,b,c$ with $a \ne 0$.
- Verify that $(a^{1/2})^2 = a$ for all $a \ne 0$.
- Give an example to prove that $(ab)^{1/2} \ne a^{1/2} b^{1/2}$ in general.