\documentclass{article}
\title{My First Document}
\author{I \and Me \and Myself}
\date{}

\begin{document}
\maketitle

\begin{abstract}
Today i.e. on \today \ we learnt a little bit of \LaTeX.
\end{abstract}

\section{First}

This is a run in formula defining $f(x_i) = \prod_{k=0}^{k=l} x^{i+1}_{i-1} +
5$.

Here is its displayed version
\[f(x_i) = \prod_{k=0}^{k=l} x^{i+1}_{i-1} + 5\]

\subsection{one} 


\begin{eqnarray}
A & = & \{ a, b, c, \} \\
B & = & \emptyset 
\end{eqnarray}



\end{document}

