%Please start the names of all of your  references and cross-references 
%with your own name!
\documentclass[a4paper,12pt]{article}
\usepackage{latexsym}
\newenvironment{proof}
 {\begin{trivlist}\item[
 {\sc Proof}]}{{\hspace{0.5pc}\phantom{b}\hfill $\Box$}
\end{trivlist}}
\newcommand{\FRAC}[2]{\displaystyle\frac{#1}{#2}}       %big fractions
\newcommand{\sR}{\mbox{\rm R\hspace{-0.7em}\rule{0.08ex}{1.55ex}\hspace{0.7em}}}
%Euclidean space, normal sized
\newcommand{\sr}{\mbox{\scriptsize \rm R\hspace{-0.7em}\rule{0.08ex}{1.55ex}\hspace{0.7em}}}
%Euclidean space, script sized
\newcommand{\sN}{\rm I\!N}%natural numbers
\newtheorem{theorem}{Theorem}[section]
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{example}[theorem]{Example}
\newtheorem{remark}[theorem]{Remark}
\newtheorem{corollary}[theorem]{Corollary}
\renewcommand\theequation{\arabic{equation}}
\renewcommand\thetheorem{\arabic{theorem}}
\newcommand{\D}{{\displaystyle}}
\newcommand{\interior}{{\rm int\ }}% interior
\newcommand{\aff}{{\rm aff\ }}% affine hull
\newcommand{\cl}{{\rm cl\ }}% closure
\newcommand{\conv}{{\rm conv\ }}%convex hull
\newcommand{\ri}{{\rm ri\ }}% relative interior
\newcommand{\cone}{{\rm cone\ }}% conical hull
\newcommand{\Argmin}[1]{\begin{array}[t]{c}{\rm Argmin}\\[-1ex]{\scriptstyle {#1}}\end{array}}
%Argmin with parameters below
\newcommand{\Argmax}[1]{\begin{array}[t]{c}{\rm Argmax}\\[-1ex]{\scriptstyle {#1}}\end{array}}
%Argmax with parameters below
\newcommand{\locmin}{{\rm locmin}\,}%local minimum
\normalbaselines
\begin{document}
\section*{insert title of the talk}
\addtocounter{section}{1}
\setcounter{theorem}{0}
\setcounter{equation}{0}
{\large\begin{center}
{\bf author1}, affiliation
%drop the following line if appropriate
\\{\bf author2}, affiliation
\end{center}}
\noindent {\bf Key Words:} Insert 2--3 key words

\noindent {\bf 2000 AMS Subject Classification:} Insert 1--2 numbers

$\sR, \sr, \sN, \cl, \Argmin{x}, \locmin, \cone$
\begin{theorem}\label{dempe 1}
\end{theorem}
\begin{proof}
\end{proof}
In Theorem \ref{dempe 1}
In Reference \cite{dempeBa98}
\begin{lemma}
\end{lemma}
\begin{theorem}
\end{theorem}
\begin{example}
\end{example}
\begin{equation}\label{dempe2}
\cl
\end{equation}
In equation (\ref{dempe2})
\begin{thebibliography}{xx}

\bibitem{dempeBa98}
J.F. Bard: {\em Practical Bilevel programming}. Kluwer, Dordrecht, 1988.

\bibitem{dempeBiKa84}
W.F. Bialas and M.H. Karwan: Two-level linear programming. 
{\em Management Science} \textbf{30,}(1984), 1004--1020.

\bibitem{dempeDem95}
S. Dempe: Discrete Bilevel Optimization Problems. Working Paper, TU Chemnitz, 1995.
\end{thebibliography}
\end{document}