\newtheorem{theo}{Theorem}[section]
\newtheorem{lem}[theo]{Lemma}
\newtheorem{pro}[theo]{Proposition}
\newtheorem{rem}[theo]{Remark}
\newtheorem{cor}[theo]{Corollary}
\newtheorem{ex}[theo]{Example}
\newtheorem{defi}[theo]{Definition}
 

%newcommand samples
\newcommand{\ot}{\otimes}
\newcommand{\co}{{\cal C}}

\title{This is the title of the paper}
\author{Firstname Lastname, second author}

\begin{document}
\date{}
\maketitle

\begin{abstract}
Here is the abstract
\end{abstract}

\noindent
1991 AMS Subject Classification:

\noindent
Keywords:
 

\section{Introduction}
\section{The second one}

\begin{defi}
A definition
\end{defi}

\begin{pro}\cite[p. 647]{aa}. Here is a Proposition
\end{pro}

\begin{ex}
An example
\end{ex}

\begin{rem}
A remark
\end{rem}

\begin{thebibliography}{99}
\small

\bibitem{aa} Name, Initial: Title, Commun. Algebra 26,
pages (1998).

\end{thebibliography}
 

\noindent
your adress, your e-mail and the date.

\end{document}