diff --git a/ca.tex b/ca.tex index 10d7946..ee98279 100644 --- a/ca.tex +++ b/ca.tex @@ -15106,13 +15106,13 @@ \subsection{Schwarz reflection principle} = 0 . \end{equation*} -In other words, the mean value equals $F(z) = 0$, +That is, the mean value equals $F(z) = 0$, and the mean-value property is satisfied for all small enough $r$ at every $z \in L$. We proved above that $F$ is harmonic on $U \setminus L$ and so the mean-value property -is satisfied for all small enough -$r$ around any of $U \setminus L$ as well. +is also satisfied for all small enough circles +around any $z \in U \setminus L$. By the mean-value property (\thmref{thm:meanprop}), $F$ is harmonic in $U$. \end{proof} @@ -15811,9 +15811,9 @@ \section{Infinite products} \label{sec:prod} If a -function has zeros at $0$, $1$, and $i$ we +function has zeros at $0$, $1$, and $i$, we can write $f$ as -$f(z) = g(z) z(z-1)(z-i)$. And if those are the only zeros, then +$f(z) = g(z) z(z-1)(z-i)$. If those are the only zeros, then $g$ is never zero. If there are infinitely many zeros, however, things become difficult. Can we factor out the zeros out of $\sin z$? Can we @@ -15825,7 +15825,7 @@ \section{Infinite products} we take care of convergence.\footnote{Beware of formal expressions bearing gifts. Especially ones with infinite sets in them. -For example, $\prod_{n \in \Z} (z-\pi n)$ does not make sense.} +For example, $\prod_{n \in \Z} (z-\pi n)$ does not actually make any sense.} Of course, we need to first figure out what we mean by convergence. Once we figure that out, we will show that convergence happens the way we want to @@ -15900,7 +15900,7 @@ \section{Infinite products} For the other direction, suppose that the sequence of partial products converges. For all but finitely many $n$, $\sabs{a_n} < 1$. -Otherwise the product would double infinitely often and go to +Otherwise the partial products would double infinitely often and go to infinity. So suppose that $\sabs{a_n} < 1$ for all $n \geq N$. If $\sabs{a_n} < 1$, then $\sabs{a_n} \leq 2 \log \bigl(1+\sabs{a_n}\bigr)$, and @@ -15917,7 +15917,7 @@ \section{Infinite products} An immediate consequence is that if $\prod_{n=1}^\infty (1+a_n)$ converges absolutely, then -$\{ a_n \}$ converges to $0$. Actually much more than that, they have to +$\{ a_n \}$ converges to $0$. Not only that, they go to zero fast enough to be absolutely summable. As for series, we need to know that absolute convergence really is convergence. @@ -15959,11 +15959,11 @@ \section{Infinite products} \\ & \leq \sum_{n=k+1}^m -\left( +\Bigl( \log \sabs{1+a_n} + \abs{ \Arg (1+a_n)} -\right) . +\Bigr) . \end{split} \end{equation} As before, @@ -15975,7 +15975,8 @@ \section{Infinite products} \end{equation*} and the series on the right converges via \propref{prop:infprodabsconvinfsum}. Next, $\Arg(1+a_n)$ is between $\nicefrac{-\pi}{2}$ and -$\nicefrac{\pi}{2}$, and $a_n$ is going to zero. So for all $n$ large +$\nicefrac{\pi}{2}$, and $a_n$ goes to zero. A calculus exercise +shows that for all $n$ large enough, $\babs{\Arg(1+a_n)} \leq 2\sabs{a_n}$. As $\sum \sabs{a_n}$ converges, $\sum \babs{\Arg (1+a_n)}$ converges. @@ -16109,9 +16110,8 @@ \section{Infinite products} \begin{exbox} \begin{exercise} -Suppose $\sabs{1+a_n} \leq r < 1$ for all -$n$, -prove that $\prod_{n=1}^\infty (1+a_n)$ converges to $0$, but that the +Suppose $\sabs{1+a_n} \leq r < 1$ for all $n$. +Prove that $\prod_{n=1}^\infty (1+a_n)$ converges to $0$, but that the convergence is not absolute. \end{exercise} @@ -17013,7 +17013,7 @@ \section{Runge's theorem} \end{exercise} \end{exbox} -Let us now approximate simple poles of the form $\frac{1}{z-p}$ by rational +We now approximate simple poles of the form $\frac{1}{z-p}$ by rational functions with poles in a given set. This procedure is called \emph{\myindex{pole pushing}} as we are going to \myquote{push} the poles along a path to where we need them to be. diff --git a/changes-draft.html b/changes-draft.html index 8a42bc7..0b78945 100644 --- a/changes-draft.html +++ b/changes-draft.html @@ -83,4 +83,5 @@