How can you number equations manually?

Question

How to number equations manually? I'm making a presentation on a chapter, so would like my equations to be numbered as in the book, and not necessarily from the first equation in the chapter, for example, if the first equation in my document is numbered (5.23). Thank you.

Answer 1

You should load the amsmath package (if you don't do so already) and use its \tag macro to assign whatever number or symbol you like to a given equation. Cross-references to such equations can be made via the usual \label--\ref (or \eqref) mechanism.

\documentclass{article}
\usepackage{amsmath}      % for \tag and \eqref macros
\setlength\textwidth{7cm} % just for this example
\begin{document}
\[
1+1=2 \tag{5.23} \label{eq:special}
\]
A cross-reference to equation \eqref{eq:special}.
\end{document}

Compile the example code twice so that LaTeX can resolve the cross-reference.

Answer 2

If your equations start at 5.23 and are not numbered sequentially, Mico's answer is certainly the right way to go. If they are numbered sequentially, however, this method is a bit error-prone, as it relies on you to keep track of the numbers. However, keeping track of the numbers (assuming they are in some predictable order) is precisely what computers are good at.

If you've got sequential equation numbering, and your only concern is to start them at a specific point, you can do this simply by resetting your counter and redefining its display:

\documentclass{article}
\begin{document}
\setcounter{equation}{22}
\renewcommand{\theequation}{5.\arabic{equation}}
\begin{equation}\end{equation}
\begin{equation}\end{equation}
\begin{equation}\end{equation}
\begin{equation}\end{equation}
\end{document}

The money bits here are the setcounter{equation}{22} and the renewcommand{\theequation}{5.\arabic{equation}} lines; the repeated empty equations just show the results:

We have to set the counter to "22" because the equation number is incremented before it is output. It's not clear to me if you're presenting the equations in the same order as in the book, but starting at a certain point; or if you're presenting the equations in a completely different order, so I thought I'd throw this answer out there in case it might be useful.