Numbering and labelling

Numbering happening automatically when you display equations. If you don't want an equation numbered, use \nonumber beside the equation. Equation numbers appear to the right of the maths by default. To make them appear on the left use the leqno class option (i.e., use \documentclass[leqno,....]{....}).

Use \label{} to label an equation (or figure, section etc) in order to reference from elsewhere.

\begin{equation}
W_{\bf S}(t,\omega) = \int\limits_{-\infty}^{\infty} {
  {\cal R}_{\bf S}(t,\tau) e^{-j\omega\tau} \,d \tau }
\label{LABELLING}
\end{equation}

$$W_{\bf S}(t,\omega) = \int\limits_{-\infty}^{\infty} { {\cal R}_{\bf S}(t,\tau) e^{-j\omega\tau} \,d \tau }$$ (3)

Now the following text

 refers back to equation \ref{LABELLING}

refers back to equation 3 by number, and

 
 refers back to the equation on page \pageref{LABELLING}

refers back to the equation on page .

A file will have to be L^ATEX'ed twice before the references, both forwards and backwards, will be correctly produced.