This is a \emph{very} simple document.
This is something written in  $math  \ mode$. It's used for in-line 
equations
such as $E = m c^2$. The displayed formula mode is for things like
\[
I = I_0 e^{\frac{e V} {k T}}
\]
and so on.
If we want numbers for our equations, we can use
\begin{equation}
F = m a
\label{eq:eg3a1}
\end{equation}
and then use the number, such as Equation~{\ref{eq:eg3a1}}
in the text.