diff --git a/ca.tex b/ca.tex index ba00616..8045eb2 100644 --- a/ca.tex +++ b/ca.tex @@ -9867,9 +9867,11 @@ \section{Zeros of holomorphic functions} an isolated zero (the only zero in fact), and $f^{(k)}(0) = 0$ for all $k=0,1,2,\ldots$. That is, the origin is a zero of infinite order. \item -Prove that for $z \in \C \setminus \{ 0 \}$, -if we define $f(z) = e^{-1/z^2}$, then the function (while holomorphic in -all of $\C \setminus \{ 0 \}$) cannot be even +Prove that +if we define $f(z) = e^{-1/z^2}$ for +$z \in \C \setminus \{ 0 \}$, +then the function, while holomorphic in +$\C \setminus \{ 0 \}$, cannot be made continuous at the origin, no matter how we'd try to define $f(0)$. \end{exparts} \end{exercise} @@ -9917,7 +9919,8 @@ \subsection{Types of singularities and Riemann extension} \label{thm:riemannext} Suppose $U \subset \C$ is open, $p \in U$, and $f \colon U \setminus \{p\} \to \C$ is holomorphic. -If $f$ is bounded (near $p$ suffices), then $p$ is a removable singularity. +If $f$ is bounded (near $p$ suffices), then $p$ is a removable singularity +of $f$. \end{thm} \begin{proof} @@ -9959,7 +9962,7 @@ \subsection{Types of singularities and Riemann extension} \end{exercise} \begin{exercise} -Suppose $U \subset \C$ is open, and +Suppose $U \subset \C$ is open and $\{ z_n \}$ is a sequence in $U$ converging to $p \in U$. Let $S = \{ z_n : n \in \N \} \cup \{ p \}$ and let $f \colon U \setminus S \to \C$ be a bounded holomorphic function. @@ -10315,7 +10318,7 @@ \subsection{Wild world of essential singularities, Casorati--Weierstrass} Let $U \subset \C$ be open, $p \in U$, and $f \colon U \setminus \{ p \} \to \C$ holomorphic. -Prove that if $f\bigl(\Delta_r(p)\bigr)$ is dense in $\C$ +Prove that if $f\bigl(\Delta_r(p) \setminus \{ p \} \bigr)$ is dense in $\C$ for all $r > 0$ such that $\Delta_r(p) \subset U$, then $f$ has an essential singularity at $p$. \end{exercise} @@ -10338,15 +10341,13 @@ \subsection{Wild world of essential singularities, Casorati--Weierstrass} \end{exercise} \begin{exercise} -\pagebreak[2] +Suppose $U \subset \C$ is open, $p \in U$, and $f \colon U \setminus \{ p \} \to \C$ +holomorphic with an essential singularity at $p$. \begin{exparts} \item -Prove a \myquote{Picard for modulus} theorem. -Suppose $f \colon U \setminus \{ p \} \to \C$ has an essential singularity -at $p \in U$. -Prove that -for every -$\Delta_r(p) \setminus \{ p \} \subset U$, the set +Prove a \myquote{Picard for modulus} theorem: +For every $r > 0$ such that +$\Delta_r(p) \subset U$, the set of all moduli of all the values of $f$ on $\Delta_r(p) \setminus \{ p \}$, that is, \begin{equation*} @@ -10362,10 +10363,10 @@ \subsection{Wild world of essential singularities, Casorati--Weierstrass} \end{exercise} \begin{exercise} -Suppose $U \subset \C$ is open and $f \colon U \setminus \{ p \} \to \C$ is -holomorphic and has -an essential singularity at $p \in U$. Then for every punctured disc -$\Delta_r(p) \setminus \{ p \} \subset U$ and every segment $[a,b] \subset +Suppose $U \subset \C$ is open, $p \in U$, and $f \colon U \setminus \{ p \} \to \C$ is +holomorphic with +an essential singularity at $p \in U$. Then for every disc +$\Delta_r(p) \subset U$ and every segment $[a,b] \subset \C$, we have $f\bigl(\Delta_r(p) \setminus \{ p \} \bigr) \cap [a,b] \not= \emptyset$. Hint: See @@ -10779,7 +10780,7 @@ \section{Residue theorem} \begin{exercise} Compute using the residue theorem -(hint: $\cos(3x) = \Re e^{i3x}$) +(hint: $\cos(3x) = \Re e^{i3x}$): \smallskip \begin{expartshor}{2} \item @@ -10802,7 +10803,7 @@ \section{Residue theorem} e^{st}F(s) \, ds \end{equation*} for some $c \in \R$ (usually $c \geq 0$) is the inverse. Compute -(using the residue theorem) +(using the residue theorem): \smallskip \begin{expartshor}{2} \item @@ -10818,7 +10819,7 @@ \section{Residue theorem} \begin{exercise} \pagebreak[2] -Compute (using the residue theorem) +Compute (using the residue theorem): \smallskip \begin{expartshor}{2} \item @@ -10829,7 +10830,7 @@ \section{Residue theorem} \end{exercise} \begin{exercise} -Suppose that $r > 1$ and $f \colon \Delta_r(0) \setminus \{ 1 \} \to \C$ is +Suppose that $r > 1$, $f \colon \Delta_r(0) \setminus \{ 1 \} \to \C$ is holomorphic, and suppose $f$ has a simple pole with $\operatorname{Res}(f;1) = 1$. If the power series for $f$ at 0 is $\sum_{n=0}^\infty c_n z^n$, show that $\lim_{n\to \infty} c_n$ exists and compute what it is. Hint: Try @@ -11045,8 +11046,7 @@ \subsection{The argument principle} \begin{exercise} Suppose $U \subset \C$ is open, $\Gamma$ is a cycle in $U$ homologous to zero in $U$, -and $f \colon U \to \C_\infty$ is meromorphic and -has no zeros or poles on $\Gamma$. +and $f \colon U \to \C_\infty$ is meromorphic with no zeros or poles on $\Gamma$. Show that there are only finitely many zeros and poles $z$ of $f$ such that $n(\Gamma;z) \not= 0$. \end{exercise} @@ -11116,8 +11116,8 @@ \subsection{The argument principle} zero holomorphic $f \colon U \to \C$ has a square root, that is, there is a holomorphic $g \colon U \to \C$ such that $g^2=f$. Hint: One direction has been proved already. For the other direction for $p \notin U$, -find a $g$ such that $g^2 = z-p$, differentiate, and apply the argument -principle. +find a $g$ such that $g^2 = z-p$, differentiate, and consider +$\Gamma$ in $U$ such that $n(\Gamma;p) = 1$. \end{exercise} \end{exbox} @@ -11461,7 +11461,8 @@ \subsection{Hurwitz's theorem} \begin{exbox} \begin{exercise} Suppose $U \subset \C$ is a domain, -$f_n \colon U \to \C$ are holomorphic and nowhere zero +and suppose +$f_n \colon U \to \C$ are holomorphic, nowhere zero, and converge uniformly on compact subsets to $f \colon U \to \C$. Show that either $f$ is nowhere zero, or $f$ is identically zero. Give examples of both possible conclusions. @@ -11470,7 +11471,7 @@ \subsection{Hurwitz's theorem} \begin{exercise} \begin{exparts} \item -Suppose $f_n \colon \D \to \C$ is a sequence converging +Suppose $f_n \colon \D \to \C$ is a sequence of holomorphic functions converging to $f \colon \D \to \C$ uniformly on compact sets such that for each $0 < r < 1$ the number of zeros (up to multiplicity) of $f_n$ in $\Delta_r(0)$ goes to infinity as $n \to \infty$. @@ -11672,7 +11673,7 @@ \section{Inverses of holomorphic functions} \label{sec:inverses} \begin{exparts} \item Show that for some neighborhood $V$ of $p$, $f|_V$ is injective. Hint: $f(z)-f(p)$ has a simple zero at $p$. -\item Show that $f(V) = W$ is open and the inverse $g \colon W \to V$ is +\item Show that $W = f(V)$ is open and the inverse $g \colon W \to V$ is continuous. \item By looking directly at the difference quotient $\frac{g(w)-g(w_0)}{w-w_0}$ show that $g$ is complex differentiable at all $w_0 \in W$. @@ -11950,9 +11951,10 @@ \subsection{Convergence of subsequences} \end{exercise} \begin{exercise} -Define a sequence of continuous functions $f_n \colon \R \to [0,1]$ that -converges pointwise to a function that is 1 on a dense set and 0 on another -dense set. Hint: Do it piecewise. +Define a sequence of continuous functions $f_n \colon \R \to [0,1]$ such +that $\{ f_n(x) \}$ converges to $1$ on a dense set of $x$ and it converges to +$0$ on another dense set. +Hint: Do it piecewise. \end{exercise} \begin{exercise}[Requires measure theory] @@ -12430,7 +12432,8 @@ \section{Montel's theorem} \end{exercise} \begin{exercise} -Prove that \myquote{locally bounded} means \myquote{bounded on compact sets,} +For open $U \subset \C$, +prove that \myquote{locally bounded} means \myquote{bounded on compact sets,} that is, $\sF$ is locally bounded if and only if for every compact $K \subset U$ there is an $M >0$ such that $\snorm{f}_K \leq M$ for all $f \in \sF$. @@ -12468,8 +12471,10 @@ \section{Montel's theorem} $f \colon \D \to \D$ such that $f(0) = 0$ and $f(\nicefrac{1}{2}) = c$. \begin{exparts} \item -Prove that $\sF_c = \emptyset$ if and only if $c \in -(\nicefrac{1}{2},1)$. +Prove that +$\sF_c = \emptyset$ if $c \in (\nicefrac{1}{2},1)$ +and +$\sF_c \not= \emptyset$ if $c \in [0,\nicefrac{1}{2}]$. \item Prove that for each $c \in [0,\nicefrac{1}{2}]$, there exists an $f \in \sF_c$ @@ -13136,6 +13141,7 @@ \subsection{Cycles around compacts and simply-connectedness}\label{subsec:pathar \begin{exercise} Suppose $\{ f_n \}$ is a sequence of holomorphic functions on an open set $U \subset \C$ that converges uniformly on compact subsets to +a nonconstant $f \colon U \to \C$. Let $K \subset U$ be a compact set. Prove that for every open neighborhood $V$ of $K$ in $U$ (so $K \subset V \subset U$) there exists a smaller open neighborhood $W$ (so $K \subset W \subset V$) and an $N \in \N$ diff --git a/changes-draft.html b/changes-draft.html index dab4f57..c606398 100644 --- a/changes-draft.html +++ b/changes-draft.html @@ -1,3 +1,7 @@ +The main point of this revision was to go through the exercises and shake out +as many typos as possible especially the exercises that weren't assigned in +my class. +