%----------preamble----------------
\makeatletter
%%%%
\renewcommand\@maketitle{%
  \newpage
  \null
  \vskip 2em%
  \begin{center}%
  \let \footnote \thanks
    {\LARGE \@title \par}%
    \vskip 1.5em%
    {\large
      \lineskip .5em%
      \begin{tabular}[t]{c}%
        \@author
      \end{tabular}\par}%
    \vskip 1em%
    {\large \@date}%
  \end{center}%
  \par
  \vskip 1.5em%
  \begin{center}%
  \abstract@produce
  \end{center}%
  \vskip 1.5em}
%
%%%
\newbox\abstractbox
\renewenvironment{abstract}{%
   \lrbox\abstractbox
     \minipage{.8\textwidth}%
     \centerline{\bfseries\large\abstractname}%
     \bigskip\small\it
  }{%
   \endminipage\endlrbox}
\def\abstract@produce{%
   \leavevmode\box\abstractbox\par
}
\makeatother
%--------------------------------------