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\begin{abstract}
In this work we ... \cite{ref1}.
 \cite{ref2} ...,
i.e.
\begin{equation}
f(x)=x^2
\end{equation}
where $x\in \mathbb{R}$.
\end{abstract}
%%THE END OF ABSTRACT

%        REFERENCES
%----------------------------
%
\begin{thebibliography}{99}

\bibitem{ref1} N. R. O. Bastos, R. A. C. Ferreira\ and\ D. F. M. Torres, Necessary optimality conditions for fractional difference problems of the calculus of variations, Discrete Contin. Dyn. Syst. {\bf 29} (2011), no.~2, 417--437.

\bibitem{ref2} N. R. O. Bastos, R. A. C. Ferreira\ and\ D. F. M. Torres, Discrete-time fractional variational problems. Signal Processing, vol. 91 (2011), 513--524.

\end{thebibliography}
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