amsthmpage-breakingtheorems
I have something like (with the packages amsthm and document class book):
\begin{proof}
blabla
\begin{itemize}
\item blabla
\item blabla
\end{itemize}
\end{proof}
The problem is, that ends on the bottom on a page and the square marking the end of the proof is alone on the next page. How can I avoid such behavior?
This can be achieved using the ntheorem package. This package handles endmarks properly. Note, you should specify the amsmath and amsthm options if you use amsmath and the proofenvironment, further thmmarks to get endmarks.
Here's a complete example, placing the qed symbol at the same height like 1-x, like desired by you:
\documentclass{article}
\usepackage{amsmath}
\usepackage[amsmath,amsthm,thmmarks]{ntheorem}
\begin{document}
\begin{proof}
\[ f(x) = \begin{cases}
2x & \text{if $x<0$}\\
7 & \text{if $x=0$}\\
1-x & \text{if $x>0$}
\end{cases} \]
\end{proof}
\end{document}
Output:
You could use further features of ntheorem to customize the proof layout and to choose a qed symbol. The ntheorem documentation may tell you more.
It is possible to "lift" the proof environment code from ntheorem:
\documentclass{article}
\usepackage[thmmarks]{ntheorem}% http://ctan.org/pkg/ntheorem
\makeatletter
\newcommand{\openbox}{\leavevmode
\hbox to.77778em{%
\hfil\vrule
\vbox to.675em{\hrule width.6em\vfil\hrule}%
\vrule\hfil}}
\gdef\proofSymbol{\openbox}
\newcommand{\proofname}{Proof.}
\newcounter{proof}\newcounter{currproofctr}\newcounter{endproofctr}%
\newenvironment{proof}[1][\proofname]{
\th@nonumberplain
\def\theorem@headerfont{\itshape}%
\normalfont
%\theoremsymbol{\ensuremath{_\blacksquare}}
\@thm{proof}{proof}{#1}}%
{\@endtheorem}
\makeatother
\theoremstyle{plain}
\theoremheaderfont{\normalfont\itshape}
\theorembodyfont{\normalfont}
\newtheorem*{myenv}{Note}
\begin{document}
\begin{myenv}
The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog.
\begin{proof}
This statement is true, because the fox did jump over the lazy dog.
\end{proof}
\end{myenv}
\end{document}
The QED-symbol is displayed when using the thmmarks package option of ntheorem. In ntheorem this is manually defined in \openbox, and subsequently used as \proofSymbol.
Best Answer
this is what the command
\qedhereis meant for withamsthm:it should be placed on the last line of the last
\item.it also works when the proof ends with display math, placed just before the
\end{...}command of the "main" environment. (there may be some disagreement over the vertical positioning in this case; for example, if the last line includescases, the box will be placed on the main baseline of that line, which is not the last line of thecasesenvironment.)