# Can I give equations the same number if they are in different chapters?

I have a document with several chapters. When I label the equations I get the chapter number and the equation label. Am I able to label an equation in a different chapter with the same label? e.g. (2.1) is the first equation in chapter 1 and (3.1) is the first equation in chapter 3.

• that numbering is usually the default. (it is in the standard book class for example) Do not use the number in the \label command if that is your question. – David Carlisle Jan 13 '16 at 16:35
• When I try to label two equations (in different chapters) e.g eq:1 and eq:1 it says that I have multiply defined labels. – Stanners Jan 13 '16 at 16:36
• do not use the number in an equation label that is supposed to be a unique descriptive identifier \label{fermat}, \label{emc2} or some such. – David Carlisle Jan 13 '16 at 16:38
• It just confuses you later if you label an equation as \label{eq:4} and then it ends up being numbered as equation 6. – David Carlisle Jan 13 '16 at 16:39
• Related/duplicate: tex.stackexchange.com/questions/278712/… – egreg Jan 13 '16 at 17:02

Labels have to be unique within the document but labels should never in any case be based on the equation number. The whole point of the \label / \ref system is that the system can renumber equations and other structures automatically without you needing to edit cross references etc. If you use a label such as \label{eq:2} it works but is confusing when the equation is numbered 3 after edits add an additional equation.

• Sorry, I hadn't realised that labelling an equation doesn't mean that is what is displayed in the final document. – Stanners Jan 13 '16 at 18:46

No... LaTeX provides the numbers automatically. Most of the time equations are numbered consecutively within e.g. chapters or sections.

If you do any math in LaTeX, you should consider the packages from the AMS (amsmath and such), they fix some blatant misfeatures of the vanilla LaTeX constructions, and offer a lot of features. See the documentation (the individual packages, perhaps with some tweaks, are included in any full-fledged TeX distribution).

If you want to reuse an equation number, e.g. because you show a specific equation for the second time, you can use \tag{\ref{eq:name}} like in

\documentclass[preview]{standalone}
\usepackage{amsmath, amssymb}
\begin{document}
$$a=b \label{eq:ab}$$
$$a=b \ \checkmark \tag{\ref{eq:ab}}$$
\end{document}