box around theorem statement

Question

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, but I do not know how to use the package.

Answer 1

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}

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}

Answer 2

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:

% 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}

Answer 3

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}

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

Answer 4

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

\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

Answer 5

If you are using thmtools, it should be noted that it can render boxed theorems per se. Example:

\documentclass{report}
\usepackage{thmtools}
\usepackage{lipsum}

\declaretheorem[
  shaded={rulecolor=black, rulewidth=1pt, bgcolor=magenta},
  name=Theorem,
]{thmboxed}

\begin{document}

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

\end{document}

renders as