# How to make proof environment without amsthm?

I want to submit a paper to this Journal and they want the author to use the etds.cls.

The class uses a package called 'theorem' and they have scripted there own theorem command

 \theorembodyfont{\itshape}
\setlength{\theorempreskipamount}{6pt plus 2pt}
\setlength{\theorempostskipamount}{6pt plus 2pt}

\begingroup \makeatletter
\gdef\th@plain{\normalfont\itshape
\def\@begintheorem##1##2{%
\def\@opargbegintheorem##1##2##3{%
\endgroup


Therefore, I can not include the package amsthm because I will get the error

! LaTeX Error: Command \theoremstyle already defined.


However, I cannot use the proof environment if I do not include the amsthm package. I asked the Journal what they would recommend and they responded that I can include a proof environment with

\newcommand{\proof}{\noindent {\it Proof. }}


but how do I get the square at the end of the proof without using amsthm?

Copy some definitions from amsthm.sty:

\documentclass{article}
%% from here-------------------------
\makeatletter
\DeclareRobustCommand{\qed}{%
\ifmmode % if math mode, assume display: omit penalty etc.
\else \leavevmode\unskip\penalty9999 \hbox{}\nobreak\hfill
\fi
\newcommand{\openbox}{\leavevmode
\hbox to.77778em{%
\hfil\vrule
\vbox to.675em{\hrule width.6em\vfil\hrule}%
\vrule\hfil}}
\newcommand{\qedsymbol}{\openbox}
\newenvironment{proof}[1][\proofname]{\par
\normalfont
\topsep6\p@\@plus6\p@ \trivlist
\item[\hskip\labelsep\itshape
#1.]\ignorespaces
}{%
\qed\endtrivlist
}
\newcommand{\proofname}{Proof}
\makeatother
%% upto here----------------------------
\begin{document}
\begin{proof}
This is a proof
\end{proof}
\end{document}


an even shorter approach, again stealing (or adapting) definitions from amsthm.sty.

\providecommand{\qedsymbol}{$\square$}
\DeclareRobustCommand{\qed}{%
\ifmmode \mathqed
\else
\leavevmode\unskip\penalty9999 \hbox{}\nobreak\hfill
\fi
}


then, if you don't really want to define a proof environment, you can simply use the \proof command suggested by the journal staff (but please use {\itshape ...} or \textit{...} instead of \it!), and end the theorem statement with

\qed


without leaving a blank line before it. this will flush the box to the right margin if it follows text, or place it a quad away from the end of the math expression if it's at the end of a display. you should also follow the proof by some extra space, say \medskip.

(personal observation on the document class -- it's woefully out of date; the documentation is dated 1999, and it wasn't even using "best practices" for that time. i'd be ashamed to distribute it.)

Quick and dirty - much shorter than @HarishKumar s solution, possibly less robust, but perhaps sufficient.

\documentclass{article}
\usepackage{amssymb}

\newenvironment{proof}
{\textit{Proof:} }
{$\square$}

\begin{document}

Theorem: whatever

\begin{proof}
Here's why.
\end{proof}
\end{document}


• This is ok, but could get orphaned from the theorem, and the final marker could also get oprhaned Jan 21, 2015 at 17:20
• Agreed, this is not a good solution (sadly seen quite a lot). Also the \textit{Proof:} part gets indented (in most cases) Jan 21, 2015 at 17:29