# How do I reference in appendix a theorem given in the body?

I have a theorem given in the body text of a paper, say Theorem 2 with label {theorem-abc}. I would like to provide the proof of this theorem in the appendix.

1) How do I cross-reference this theorem in the appendix section title?

The section title that I want is "Proof of Theorem 2".

2) How do I restate the theorem again in the aforementioned section?

What I want is "Theorem 2. xxxxxxx", followed by its proof.

ADD: The document class file has following definitions regarding Appendix.

\def\appendix{\par
\section*{APPENDIX}
\setcounter{section}{0}
\setcounter{subsection}{0}
\def\thesection{\Alph{section}} }

I found similar issues here. But it did not solve my Problem 1) either.

\documentclass{vldb}
\usepackage{thmtools, thm-restate}
\usepackage{hyperref}
\declaretheorem{theorem}

\begin{document}
\section{Test Section}
\begin{theorem}
Test theorem
\end{theorem}

\begin{restatable}[Fundamental Theorem of Algebra]{theorem}{fta}
\label{thm:FTA}
Every non-constant single-variable polynomial with complex coefficients has at least    one complex root.
\end{restatable}

\appendix
\section[Proof of the Fundamental Theorem of Algebra]{Proof of Theorem~\ref{thm:FTA} (See page~\pageref{thm:FTA})}
\fta*
\begin{proof}
Assume...
\end{proof}

\end{document}
-
A minimal working example (MWE) would, among other things, reveal which class you're using. –  lockstep May 14 '13 at 10:16

2- You can use the restatable environment provided by thmtools.

1- Since the environment generates an anchor, you can use the standard \label, \ref (\pageref) mechanism for cross-referencing.

A complete example:

\documentclass{article}
\usepackage{amsthm}
\usepackage{thmtools, thm-restate}
\usepackage{hyperref}

\declaretheorem{theorem}

\begin{document}

\section{Test Section}
\begin{theorem}
Test theorem
\end{theorem}

\begin{restatable}[Fundamental Theorem of Algebra]{theorem}{fta}
\label{thm:FTA}
Every non-constant single-variable polynomial with complex coefficients has at least one complex root.
\end{restatable}

\appendix
\section[Proof of the Fundamental Theorem of Algebra]{Proof of Theorem~\ref{thm:FTA} (See page~\pageref{thm:FTA})}
\fta*
\begin{proof}
Assume...
\end{proof}

\end{document}

Now that the question has been edited, and the used document class is known, here's a solution using the vldb document class (the link to the class can be found in the question):

\documentclass{vldb}
\usepackage{textcase}
\usepackage{thmtools,thm-restate}
\usepackage{hyperref}

\makeatletter
\def\@sect#1#2#3#4#5#6[#7]#8{%
\ifnum #2>\c@secnumdepth
\let\@svsec\@empty
\else
\refstepcounter{#1}%
\edef\@svsec{%
\begingroup
%\ifnum#2>2 \noexpand\rm \fi % changed to next 29 July 2002 gkmt
\ifnum#2>2 \noexpand#6 \fi
\csname the#1\endcsname
\endgroup
\ifnum #2=1\relax .\fi
\hskip 1em
}%
\fi
\@tempskipa #5\relax
\ifdim \@tempskipa>\z@
\begingroup
#6\relax
\@hangfrom{\hskip #3\relax\@svsec}%
\begingroup
\interlinepenalty \@M
\MakeTextUppercase{#8}% HERE
\else
#8%
\fi
\par
\endgroup
\endgroup
\csname #1mark\endcsname{#7}%
\vskip -12pt  %gkmt, 11 aug 99 and GM July 2000 (was -14) - numbered section head spacing
\ifnum #2>\c@secnumdepth \else
\protect\numberline{\csname the#1\endcsname}%
\fi
#7%
}%
\else
\def\@svsechd{%
#6%
\hskip #3\relax
\@svsec
\uppercase{#8}%
\else
#8%
\fi
\csname #1mark\endcsname{#7}%
\ifnum #2>\c@secnumdepth \else
\protect\numberline{\csname the#1\endcsname}%
\fi
#7%
}%
}%
\fi
\@xsect{#5}\hskip 1pt
\par
}
\makeatother

\declaretheorem{theorem}

\begin{document}

\section{Test Section}
\begin{theorem}
Test theorem
\end{theorem}

\begin{restatable}[Fundamental Theorem of Algebra]{theorem}{fta}
\label{thm:FTA}
Every non-constant single-variable polynomial with complex coefficients has at least one complex root.
\end{restatable}

\appendix
\section[Proof of the Fundamental Theorem of Algebra]{Proof of Theorem~\ref{thm:FTA}}
\fta*
\begin{proof}
Assume...
\end{proof}

\end{document}

## Explanation:

The problem is that vldb.cls sets the section (and also part) titles in upper case and uses the old TeX \uppercase command to this end; this will produce problems if special commands, such as \ref, are inside the argument of \section (or \part) since \uppercase don't recognize them as special commands and tretas them in the wrong way (the LaTeX command \MakeUppercase will also behave bad in this regard). To correct this situation, one can redefine (or patch) the internal command \@sect, to use \MakeTextUppercase (from the textcase package), instead of the problematic \uppercase command. In the example code above, the change was signaled with % HERE.

-
I followed your example, but encountered a problem. "2" in "Proof of Theorem 2" appeared as "?" (I had tried compiling the document many times). Would this be an issue of the document class file? –  jason.Z May 14 '13 at 9:47
@jason.Z please see my updated answer. –  Gonzalo Medina May 14 '13 at 13:04
Problem solved, and thanks a lot! –  jason.Z May 14 '13 at 23:05