# Problem with proof environment

I've written a proof in LaTeX, and I looked up how to use the proof environment. From what I read, it should start with "Proof:" and end with \qed, but the PDF shows no changes. It simply displays the text verbatim. What could be the cause of this? My code is as follows:

\documentclass[12pt]{report}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{tikz}
\usepackage{graphicx}
\usepackage{float}
\usepackage{framed}
\usepackage[hang,flushmargin]{footmisc}

\newtheorem{theorem}{Theorem}[section]
\newtheorem{corollary}{Corollary}[theorem]
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{definition}{Definition}[section]

\begin{document}
\begin{proof}
\noindent Assume that $R$ decides $HALT_{TM}$, and obtain a contradiction. Construct $S$ to decide $A_{TM}$, where $S$ operates as follows: \newline \newline
$S=$ On input $\langle M, w \rangle$, an encoding of a TM $M$ and a string $w$:
\begin{enumerate}
\item Run TM $r$ on input $\langle M, w \rangle$.
\item If $R$ rejects, reject
\item If $R$ accepts, accept
\item If $M$ has accepted, accept; if $M$ has rejected, reject."
\end{enumerate}

\noindent \newline If $R$ decides $HALT_{TM}$, then $S$ decides $A_{TM}$. Because $A_{TM}$ is undecidable, $HALT_{TM}$ must also be undecidable.
\end{proof}
\end{document}

• please fix your example to be a complete document that shows the problem. proof isn't defined by default but several packages define environments of this form so it is impossible to guess what definitions you are using. Commented Jul 5, 2018 at 23:26
• Do you require a picture of the PDF? If so, how do I do that? Commented Jul 5, 2018 at 23:30
• as posted the document produces the error ! LaTeX Error: Environment proof undefined. after an error you shouldn't really even look at the pdf, you need to fix the error first, Commented Jul 5, 2018 at 23:33
• I'm not getting any errors. That's why I'm confused. Commented Jul 5, 2018 at 23:34
• did you intend to load the amsthm package? Commented Jul 5, 2018 at 23:34

## 1 Answer

The proof environment is not defined so you get the error

! LaTeX Error: Environment proof undefined.


a common way to define it is to load amsthm when you get a proof heading and qed box at the end.

\documentclass[12pt]{report}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{tikz}
\usepackage{graphicx}
\usepackage{float}
\usepackage{framed}
\usepackage[hang,flushmargin]{footmisc}

\usepackage{amsthm}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{corollary}{Corollary}[theorem]
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{definition}{Definition}[section]

\begin{document}
\begin{proof}
Assume that $R$ decides $\mathrm{HALT}_{\mathrm{TM}}$, and obtain a contradiction. Construct $S$ to decide $A_{\mathrm{TM}}$, where $S$ operates as follows:

$S={}$ On input $\langle M, w \rangle$, an encoding of a TM $M$ and a string $w$:
\begin{enumerate}
\item Run TM $r$ on input $\langle M, w \rangle$.
\item If $R$ rejects, reject
\item If $R$ accepts, accept
\item If $M$ has accepted, accept; if $M$ has rejected, reject."
\end{enumerate}

If $R$ decides $\mathrm{HALT}_{\mathrm{TM}}$, then $S$ decides $A_{\mathrm{TM}}$. Because $A_{\mathrm{TM}}$ is undecidable, $\mathrm{HALT}_{\mathrm{TM}}$ must also be undecidable.
\end{proof}
\end{document}

• Thank you! I guess ShareLaTeX is missing some info. At least, from what I could see. Commented Jul 5, 2018 at 23:52
• @user83024 i am sure that sharelatex is flagging errors if you look in the right place, also it can show you the log file which will also have the error message. I can't give details as I don't use it myself Commented Jul 5, 2018 at 23:56