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Is there an easy way to put a box around a theorem in LaTeX? For example to state an important theorem.

I tried using page 20 of the ntheorem documentation (http://ctan.mackichan.com/macros/latex/contrib/ntheorem/ntheorem.pdf), but I do not know how to use the package.

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ntheorem uses pstricks to shade theorem. So you have to compile via latex-ps-pdf or xelatex. -- Maybe the package framed is helpful. –  Marco Daniel Nov 26 '11 at 23:43

4 Answers 4

up vote 14 down vote accepted

You can use \newmdtheoremenv from the mdframed package:

\documentclass{article}
\usepackage{mdframed}
\usepackage{lipsum}

\newmdtheoremenv{theo}{Theorem}

\begin{document}

\begin{theo}
\lipsum*[1]
\end{theo}

\end{document}

enter image description here

If you only want to frame some theorems, then you can define a new environment using the mdframed environment and some previously defined theorem-like environment:

\documentclass{article}
\usepackage{mdframed}
\usepackage{lipsum}

\newtheorem{theo}{Theorem}
\newenvironment{ftheo}
  {\begin{mdframed}\begin{theo}}
  {\end{theo}\end{mdframed}}

\begin{document}

\begin{ftheo}
\lipsum*[1]
\end{ftheo}

\begin{theo}
\lipsum*[1]
\end{theo}

\end{document}
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Thank you for the effort.For some reason I don't get the whole box: I miss the bottom and left line. –  Nadori Nov 27 '11 at 0:05
    
@Nadori are you sure it's not only a viewer issue? –  Gonzalo Medina Nov 27 '11 at 0:11
    
@Gonzalo: How do you mean? But if I replace the theorem I get all the sides of the box except the left side. –  Nadori Nov 27 '11 at 0:19
1  
@Nadori: No there are no problems ;-) –  Marco Daniel Nov 27 '11 at 0:26
4  
@Nadori: I noticed that you've received some good answers to your other questions, but you haven't accepted any of them. Please consider marking those answers that you consider to be the best as ‘Accepted’ by clicking on the tickmark below their vote count. This shows which answer helped you most, and it assigns reputation points to the author of the answer (and to you!). –  Gonzalo Medina Nov 27 '11 at 0:36

Here's an example of how you can put some colour into your Theorems using ntheorem combined with PSTricks.

screenshot

\documentclass{article}

\usepackage{pstricks}
\usepackage{pst-grad}
\usepackage{framed}
\usepackage[framed]{ntheorem} 
\usepackage{lipsum}

% framed theorem, red background
\theoremstyle{plain}
\theoremsymbol{}
\theoremseparator{}
\def\theoremframecommand{%
      \psshadowbox[fillstyle=solid,fillcolor=red,linecolor=black]}
\newshadedtheorem{mytheorem}{Special Theorem}

% framed theorem, gradient shading
\theoremstyle{plain}
\theoremsymbol{}
\theoremseparator{}
\def\theoremframecommand{%
      \psshadowbox[fillstyle=gradient,gradbegin=red,gradend=yellow,linecolor=black]}
\newshadedtheorem{myfancytheorem}{Sunset special theorem}

\begin{document}

\begin{mytheorem}
\lipsum[1] 
\end{mytheorem}

\begin{myfancytheorem}
 \lipsum[1]
\end{myfancytheorem}

\end{document}

Either run the above code with

latex myfile.tex
dvips myfile.dvi
ps2pdf myfile.ps

or

xelatex myfile.tex

If you want to run it with pdflatex then add the option pdf to the pstricks load

\usepackage[pdf]{pstricks}

and compile with

pdflatex -shell-escape myfile.tex
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I don't know if the tcolorbox package was available at the time that the question was asked, but here's a small example taken directly from the documentation:

screenshot

% arara: pdflatex
\documentclass{article}

\usepackage{tcolorbox}
\tcbuselibrary{theorems}

\newtcbtheorem[number within=section]{mytheo}{My Theorem}%
{colback=green!5,colframe=green!35!black,fonttitle=\bfseries}{th}

\begin{document}
\begin{mytheo}{This is my title}{theoexample}
  This is the text of the theorem. The counter is automatically assigned and,
  in this example, prefixed with the section number. This theorem is numbered with
  \ref{th:theoexample} and is given on page \pageref{th:theoexample}.
\end{mytheo}

\end{document}
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Here's a minimum working example (MWE) of how one can use the ntheorem and framed packages to draw a rectangular frame around a theorem environment:

\documentclass{article}
\usepackage{framed} % or, "mdframed"
\usepackage[framed]{ntheorem}
\newframedtheorem{frm-thm}{Theorem}
\begin{document}
\begin{frm-thm}[Pythagoras]
Let $a$, $b$, and $c$ denote the lengths of the sides of a \emph{right 
triangle}, i.e., of a triangle with one angle equal to $90^\circ$. 
Without loss of generality assume that $a\le b<c$. Then
\[ a^2+b^2=c^2. \]
\end{frm-thm}
\end{document}

enter image description here

Check the user guides of the framed and mdframed packages for available options for setting the style of the frame.

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