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How would I define a proof environment that is basically used with

\begin{proof}
some proof here
\end{proof}

and it would be equivalent to:

\paragraph{Proof:} some proof here
\hfill \box

?

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You probably want to add a “qed” box at the end of a proof. For this, it is enough to load a package such as amsmath. –  Juan A. Navarro Feb 3 '11 at 7:25
7  
Environments for proofs, theorems etc. are provided by either the amsthm or the ntheorem package. –  Carsten Thiel Feb 3 '11 at 8:34

4 Answers 4

up vote 11 down vote accepted

With LaTeX syntax:

\newenvironment{proof}{\paragraph{Proof:}}{\hfill$\square$}

I assume here that you mean \hbox{} not \box in your code. The \null macro is short for \hbox{}. The \box is a TeX primitive and is more like \usebox.

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2  
this "box" at the end is quite invisible. and if the proof ends with a display or a list, there's an extra blank line. –  barbara beeton May 21 '13 at 21:29
    
@barbarabeeton: It was not my original intension to draw a real box there because I took the \box macro of the OP with it's normal TeX meaning ;-) I modified the answer now to draw a real box. I don't have a quick fix for blank line issue. IMHO Someone should post an answer based on amsthm or a similar package. –  Martin Scharrer May 22 '13 at 6:44
    
This solution gave me an extra blank line at the beginning. Replacing \paragraph{Proof:}} with \textbf{Proof:} solved the problem. –  ntc2 Jan 28 at 4:46
    
@ntc2, I'm guessing that was an indent from \paragraph{} which can be suppressed with \noindent –  PatrickT Feb 1 at 22:22
\def\QEDmark{\square}
\def\proof{\paragraph{Proof:}}
\def\endproof{\hfill\QEDmark}
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this one seems to be giving an error about \begin{document} missing whenever I try to use \begin{proof} \end{proof} –  kloop Feb 2 '11 at 23:46
    
@kloop: I think it's the problem Martin pointed out - \box is a Tex primitive that looks ahead for the box number, and I guess then will eat the closing parenthesis. I've changed the code. –  Charles Stewart Feb 2 '11 at 23:58
    
@kloop: BTW, you can vote up as well as accept Martin's answer. –  Charles Stewart Feb 2 '11 at 23:59
    
Aaahhh, he meant to draw a box using \box, ok ... I sometimes I think to complicated. –  Martin Scharrer Feb 3 '11 at 0:20
    
The \square should be enclosed in $ signs: \def\QEDmark{$\square$}. –  user31011 May 21 '13 at 3:10

This is a solution using the ntheorem package. Here some explanations:

  • amssymb is loaded to use \blacksquare
  • The package ntheorem is loaded using the option thmmarks to support the placement of endmarks.
  • \theoremheaderfont{\bfseries} sets the theorem title in bold.
  • \theorembodyfont{\normalfont} preserves the normal text font for the theorem content.
  • \theoremseparator{:} set : to be the separator between title and content.
  • \theoremsymbol{$\blacksquare$} will place a solid black square at the end of every theorem environment.
  • \newtheorem*{proof}{Proof} sets up a new unnumbered environment named proof with the default title "Proof".

Afterwards you can use \begin{proof} ... \end{proof} just like any other environment.

Implementation

\documentclass{article}
\usepackage{amssymb}
\usepackage[thmmarks]{ntheorem}
\theoremheaderfont{\bfseries}
\theorembodyfont{\normalfont}
\theoremseparator{:}
\theoremsymbol{$\blacksquare$}
\newtheorem*{proof}{Proof}
\begin{document}
\begin{proof}
    We immediately see, that
    \[ 1 + 1 = 2. \]
    Thus the proof is done.
\end{proof}
\end{document}

Output

image

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the box won't be there if the proof ends with a display or a list. regarding ending with a display, that's not great style; better to finish off with text as in this example. but a list might be harder to avoid. –  barbara beeton May 21 '13 at 21:23
    
Actually in some case the box will be there when ending with a dislay (although the order of appearance seems very random to me). Nevertheless I agree with you on the style issue. –  Henri Menke May 21 '13 at 21:37
    
The claimed output is not the one I get when i compile the given code. –  anderstood Oct 7 at 15:57
    
OK I see: the figure is wrapped in a SE environment... –  anderstood Oct 7 at 15:58

You can also use the amsthm package which is a really nice and quick way of creating a proof.

Implementation

\documentclass{article}
\usepackage{amsthm}

\begin{document}
\begin{proof}
    Your proof here.
\end{proof}

\end{document}

It will add Proof in italics at the beginning of the text given as argument and a white square (Q.E.D. symbol) at the end of it.

I found this easy solution at 1. You can also look up there how you define your own environments using this package.

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amsthm's proof has the (known) flaw of introducing too much space when the proof directly starts with an enumerate or itemize environment. –  Marius Hofert Apr 26 at 18:55

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