\documentclass{article}        % Your input file must contain these two lines 
\begin{document}               % plus the \end{document} command at the end.

\section{Mathematical Formulas}
\label{math}

You can see the power of \LaTeX when you have to type a complicated formula:
\begin{equation}
\sum_{i=1}^{n} \frac{a^i}{b^i} + \frac{c_i}{d^i} 
\end{equation}
\label{add}

Here's how they look when displayed:
\begin{equation}
\sum_{i=1}^{n} x_{i} = \int_{0}^{1} f
\label{integral}
\end{equation}
and in the text:
\(\sum_{i=1}^{n} x_{i} = \int_{0}^{1} f\).

\section{Cross-References}

One reason for numbering things like figures and equations is to refer the reader to them, as in: ``See Equation~\ref{add} 
and Equation~\ref{integral} 
in Section~\ref{math}.''
You don't want the ``2'' to appear in the input file because adding another equation will make it become Figure 3.

\subsection{Citation}

A citation is a cross-reference to another publication, such as a journal article, called the \emph{source}.

The modern method of citing a source is with a cross-reference to an entry in a list of sources at the end of the document.  With \LaTeX, the citation is produced by a \texttt{$\backslash$cite} command having the citation key as its argument.


According to our survey of one-time password, the technique proposed in \cite{lamport} is used in RFC 2289~\cite{rfc2289}.

\begin{thebibliography}{99}
\bibitem{lamport} Leslie Lamport, ``Password Authentication with Insecure
        Communication'', {\em Communications of the ACM}, November
        1981), pp.770-772.
\bibitem{rfc2289}  N. Haller, C. Metz, P. Nesser, M. Straw, ``A One-Time Password System'', {\em IETF RFC 2289}, February 1998.
\end{thebibliography}

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