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In many papers in mathematics, the author states a theorem in somewhere and adds some explanations and perhaps some lemmas and later starts the proof with a phrase like "Proof of Theorem x.xx" (x's stand for some numbers). The only format I am aware for starting and ending a proof is as follows:

\begin{proof}

\end{proof}

This format is good when I start the proof immediately after stating theorems (propositions etc). So my question is:

How can I start the proof later with a phrase like "Proof of Theorem x.xx"?

P.S. I am not sure about the tags I have used. Please, modify them if you know better tags.

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2  
What are you using to create your theorem and proof environments? –  Werner Sep 26 '13 at 19:03
    
I wrote how I create my proofs. I create theorems with similar commands. Except I add a label after begin; \begin{theorem} \label{thm:somelabel} ... the statement of the theorem ... \end{theorem} –  Vahid Shirbisheh Sep 26 '13 at 19:07
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1 Answer

up vote 11 down vote accepted

Assuming you're using a package such as amsthm, you could proceed as follows: Assign a \label to the theorem, and then cross-reference it via a \ref statement at the start of the associated proof environment.

enter image description here

\documentclass{article}
\usepackage{amsthm}
\usepackage{lipsum} % for filler text
\newtheorem{theorem}{Theorem}
\begin{document}

\begin{theorem}[Pythagoras] \label{thm:neat}
$a^2+b^2=c^2$.
\end{theorem}

\lipsum[2]

\begin{proof}[Proof of Theorem \ref{thm:neat}]
(state your proof here)
\end{proof}

\end{document}

Addendum: For comparison, here's the output of the same MWE (minus the filler text) if you were to use the ntheorem package. (Observe that ntheorem doesn't automatically place a QED symbol at the end of a proof environment.)

enter image description here

\documentclass{article}
\usepackage{ntheorem}
\newtheorem{theorem}{Theorem}
\theoremstyle{empty}
\newtheorem{refproof}{Proof}
\begin{document}
\begin{theorem}[Pythagoras] \label{thm:neat}
$a^2+b^2=c^2$.
\end{theorem}

\begin{refproof}[Proof of Theorem \ref{thm:neat}]
(state your proof here)
\end{refproof}
\end{document}
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Thank you It's exactly the answer I was looking for. I assume that I can write any thing instead of "proof of Theorem", for example "the second part of the proof of Theorem", and so on. –  Vahid Shirbisheh Sep 26 '13 at 19:15
2  
@VahidShirbisheh - You could definitely have both \begin{proof}[First part of the proof of Theorem \ref{thm:neat}]... and, later on, \begin{proof}[Second part of the proof of Theorem \ref{thm:neat}].... Of course, you'll have to replace thm:neat` with the actual label of the theorem in question. –  Mico Sep 26 '13 at 21:47
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