# macro for the theorem environment

It is a pain to type

\begin{theorem}
...
\end{theorem}

\begin{proof}
...
\end{proof}


when live texing a document in class.

When I declare

\newcommmand{\theorem}[2]{\begin{theorem} {#1} \begin{proof} {#2} \end{proof} \end{theorem}}


I get that \theorem is already defined.

If I try

\renewcommmand{\theorem}[2]{\begin{theorem} {#1} \begin{proof} {#2} \end{proof} \end{theorem}}


I get a fatal error.

Is there any way to prevent myself from having to type the words beginning, end and proof several times? This is important when I am live texing notes in class.

• This is mainly a problem with the editor. Most editors nowadays have syntax completion and macros that allow to type environments with a simple shortcut. – Bernard Jan 18 '17 at 21:53
• or you could make a new command with a custom name, like \newcommand{\mythm} – Marijn Jan 18 '17 at 21:54
• You're shooting to your own foot. Don't do that. – egreg Jan 18 '17 at 21:56

You can do it with

\newcommand{\thmpr}[2]{%
\begin{theorem}#1\end{theorem}%
\begin{proof}#2\end{proof}%
}


but I warn you that this is much worse: you need to keep track of braces that can end up very far away from each other.

If you're taking class notes, it's much easier to tag:

THEOREM\\
Whatever the guy at the blackboard is saying

PROOF\\
Something even more mysterious that I'll go through later


It's then easy to place the required \begin and \end tag when you revise the material.

The problem is that \newtheorem{theorem}{Theorem} does define a \theorem macro for its internal purposes. So LaTeX refuses to do \newcommand{\theorem}[2]{...}; but with

\renewcommand{\theorem}[2]{\begin{theorem}...}


you're defining \theorem in terms of itself and this will cause an infinite loop.

• Isn't this basically my answer? – Ruben Jan 18 '17 at 22:07
• @Ruben The explanation of the error is similar. Your proposed workaround is useless. – egreg Jan 18 '17 at 22:10
• I think not showing the whole workaround does not make it useless. Nontheless I added the last bit. – Ruben Jan 18 '17 at 22:17

The problem in your definition is that \begin{theorem} implicitly calls \theorem. Thats why in your first attempt you get an 'alredy-defined'-error. (The environment in the end is really just that macro.) In the second try you call your macro inside the one you want to redefine. Hence you end up in an infinite loop. Try to store a version of \theorem by saying

\let\oldtheorem\theorem


and then your second attempt basically should work:

\documentclass{article}
\usepackage{amsthm}

\newtheorem{theorem}{Theorem}
\let\oldtheorem\theorem
\let\endoldtheorem\endtheorem
\renewcommand\theorem[2]{\begin{oldtheorem}#1\end{oldtheorem}\begin{proof}#2\end{proof}}

\begin{document}
\theorem{a theorem}{with proof}
\end{document}


Also note that the proof environment usually does not get nested inside the theorem and the grouping around the arguments, i.e. '{#1}' is needless.

• I think it is really not the preferred way of marking up the document, but the OP described a particular case and first of all I tried to explain the misbehaviour. In this specific case I think it is good advice actually. – Ruben Jan 18 '17 at 22:03
• Seen edit, now it’s much better! But I still prefer @egreg’s answer. – GuM Jan 18 '17 at 22:15
• @GustavoMezzetti -- where does it differ exactly? (the only difference is the naming of the command.) – Ruben Jan 18 '17 at 22:16
• I prefer @egreg’s answer exactly and solely because it recommend using a different name for the command: I think that reusing the same name isn’t good practice. Edit: Well, actually there’s also the issue pointed out by D.C., but that one is easy to correct! :-) – GuM Jan 18 '17 at 22:20