\[
\newcommand{\Ann}{\mathsf{Ann}}
\newcommand{\Arg}{\mathsf{Arg}}
\newcommand{\C}{\mathbb{C}}
\newcommand{\R}{\mathbb{R}}
\newcommand{\N}{\mathbb{N}}
\newcommand{\Z}{\mathbb{Z}}
\newcommand{\Im}{\mathsf{Im}}
\newcommand{\intd}{\,\mathsf{d}}
\newcommand{\Re}{\mathsf{Re}}
\newcommand{\Res}{\mathsf{Res}}
\newcommand{\ball}{\mathsf{B}}
\newcommand{\wind}{\mathsf{wind}}
\newcommand{\Log}{\mathsf{Log}}
\newcommand{\l}{<}
\]
Week 10 Worksheet
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Poles and Residues
- For each of the following functions determine its poles, their orders, and their residues.
- $\dfrac{1}{z(1-z^2)}$ on $\C \setminus \{0,1,-1\}$
- $\dfrac{\sin(z)}{\cos(z)}$ on $\C \setminus (\pi \Z + \frac{\pi}{2})$
- $\dfrac{z}{1+z^4}$ on $\C \setminus \{e^{i \pi / 4}, e^{3i \pi / 4}, e^{5 i \pi / 4}, e^{7 i \pi / 4} \}$
- $\left( \dfrac{z+1}{z^2 + 1} \right)^2$ on $\C \setminus \{i,-i\}$
- Determine the singularities of the following functions and use Taylor series to calculate their residues.
- $\dfrac{\sin(z)}{z^2}$
- $\dfrac{(\sin z)^2}{z^4}$
- Define $f : \C \setminus \{0,1\} \to \C$ by $f(z) = \dfrac{1}{z (1-z)^2}$. It has singularities at $0$ and $1$.
- Determine the Laurent series of $f$ on $\Ann(0,0,1)$ and $\Ann(1,0,1)$.
- Use your answers to write down the orders of any poles and the residues of the singularities.
- Verify your answers to b) using the lemmas from the video "Calculating Residues".
- Define
\[
f(z) = \left( \dfrac{z+1}{z-1} \right)^3
\]
on $\C \setminus \{1\}$. Calculate $\Res(f,1)$ by determining the Laurent series of $f$ on $\Ann(1,0,\infty)$.
- Determine the residue of $\dfrac{1}{z^2 \sin(z)}$ at zero.
- Suppose $f,g : D \to \C$ are holomorphic and $b \in D$. Assuming $f$ has a zero of order $m$ at $b$ and $g$ has a zero of order $n$ at $b$ prove that $f(z) g(z)$ has a zero of order $n+m$ at $b$.
Cauchy's Residue Theorem
- Given $r > 0$ write $\gamma_r(t) = re^{it}$ on $[0,2\pi]$.
Use Cauchy's residue theorem to evaluate each of the following integrals.
- $\displaystyle\int\limits_{\gamma_4} \dfrac{1}{z^2 - 5z + 6} \intd z$
- $\displaystyle\int\limits_{\gamma_{5/2}} \dfrac{1}{z^2 - 5z + 6} \intd z$
- $\displaystyle\int\limits_{\gamma_2} \dfrac{\exp(az)}{1 + z^2} \intd z$ where $a \in \R$