# Using theorem environment but theorems not displayed properly

I am using a class file aptpub.cls (Applied Probability, it can be found here) and one of the suggestions for an environment for theorems in that class file is \newtheorem{theorem}{\noindent Theorem}. I am also using amsmath package. So if I use the aforementioned environment for a theorem and if the code for the theorem is

\documentclass{aptpub}
\usepackage{amsthm}
\usepackage{amsfonts}
\usepackage{mathtools}
\newtheorem{theorem}{\noindent Theorem}

\begin{document}
\begin{theorem}
\label{t1}
abc=xyz
\end{theorem}

\end{document}


then it is printed in pdf file as:

May I know how it can be corrected? Thanks.

• Welcome to TeX.SE! Please provide a full minimal working example which reproduces the issue, starting with \documentclass{...} and ending with \end{document}. In this way we can copy-paste the code and look at the problem without resorting to wild guesses about what you are doing. Furthermore, since aptpub.cls is no standard class you might want to add a link to it. Jul 27 at 13:19
• @campa, I have edited the question now. Thanks. Jul 27 at 13:25
• @egreg Added the code now. Jul 27 at 13:27

The class is incompatible with amsthm because it uses the (obsolete) package theorem and already defines a theorem environment.

So just do

\documentclass{aptpub}
\usepackage{amsfonts}
\usepackage{mathtools}

\begin{document}

\begin{theorem}\label{t1}
abc=xyz
\end{theorem}

\end{document}


and you'll be on the right track.