# Choosing numbering on equations

I'm writing some course notes and through out I want to refer to the equation number in the book we are using, rather than numbering the equations within the document. In other words how can I decide for myself what the equation number should be? I've made a MWE below, but the problem is that it puts the equation number on the next line below the equation.

\documentclass[11pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\begin{document}
The density of states at $\epsilon$ is
\begin{center}
$D_{e}(\epsilon)=\dfrac{1}{2\pi^{2}}\left(\dfrac{2m_{e}} {\hbar^{2}}\right)^{3/2}\left(\epsilon- E_{c}\right)^{1/2}$\end{center}\begin{flushright}\textrm{(8.37)}\end{flushright}
\end{document}


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You can use \tag and an equation environment:

\documentclass[11pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\begin{document}
The density of states at $\epsilon$ is
$$D_{e}(\epsilon) = \frac{1}{2\pi^{2}}\biggl( \dfrac{2m_{e}} {\hbar^{2}} \biggr)^{3/2} (\epsilon - E_{c} )^{1/2} \tag{8.37}$$
\end{document}


Notice also that I didn't use the \left, \right construct but the \biggl, \biggr commands instead.

If you have access to the original .tex document in which the equation appears, you could use the xr package to help you cross-reference the external expressions.

You could be interested in the following documents to help you typeset math expressions:

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