[Tex/LaTex] Cross-referencing an equation

cross-referencingequations

So far I've always used non-labeled alignments with align*, but I think it's time for a small change. How do I refer to an alignment after creating it? I tried searching similar topics but I didn't find them here. I am using article document class.

Also, does equation behave and work similarly to align? And how would you refer to an equation number (same situation as with align)?

Best Answer

Your question is more a general question about labeling and referencing equations.

I think one great introduction is the mathmode of Herbert Voss. It's available at CTAN.

mathmode at CTAN

In the linked document you will find an extra sub section about Labels. It is introduced by:

Every numbered equation can have a label to which a reference is possible.

There is one restriction for the label names, they cannot include one of LaTeX’s 8 command characters.

The label names are replaced by the equation number.

Update

The following point was mentioned by Mico:

It may be useful to note that in addition to the eight "basic" special characters that can't be used inside labels, users should also refrain from using

commas if the plan on using the cleveref package or

any characters that have special meanings for various languages supported by the babel package (such as : in babel/French).

The environment equation doesn't allow any line breaks. So the syntax will be:

\begin{equation}
 a^2+b^2=c^2\label{eq:1}
\end{equation}

The environment align allows line breaks and so every line can get a label.

\begin{align}
  x^2+y^2&=2r^2 \label{eq:1} \\
  d^2+h^2&=4r^2 \label{eq:2}
\end{align}

To reference to a given label you can use the standard command \ref or any other reference command provided by some packages. I like to use \eqref to reference to equations.

Here a complete example:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
 a^2+b^2=c^2\label{eq:1}
\end{equation}
Text \eqref{eq:1}
\end{document}

Document A

The first document test-A.tex writes the latest value of counter equation in the zref reference eq-last:

% test-A.tex
\documentclass{article}

\usepackage{zref-base}
\usepackage{atveryend}
\makeatletter
\AfterLastShipout{%
  \if@filesw   
    \begingroup
      \zref@setcurrent{default}{\the\value{equation}}%
      \zref@wrapper@immediate{%
        \zref@labelbyprops{eq-last}{default}%
      }%
    \endgroup
  \fi
}
\makeatother

\begin{document}
\begin{equation}E=mc^2\end{equation}
\end{document}

Remarks:

Instead of \AtEndDocument it uses the hook \AfterLastShipout that is executed after the last page is shipped out and before the .aux file is closed.
\zref@wrapper@immediate maps \write to \immediate\write in order to write into the .aux file directly. At this point deferred writing without \immediate will not work, because the last page is already shipped out and there is no next page.
The code is put inside \if@filesw and \fi. That supports \nofiles, that forbids writing to auxiliary files.

Document B

The next "book in the series" imports the references from document test-A. The reference names are prefixed with A- to avoid name clashes (\zexternaldocument):

% test-B.tex
\documentclass{article}

\usepackage{zref-xr}
\makeatletter
\zexternaldocument[A-]{test-A}
\AtBeginDocument{%
  \setcounter{equation}{\zref@extractdefault{A-eq-last}{default}{0}}%
  \zref@refused{A-eq-last}%
}
\makeatother

\begin{document}
\begin{equation}v=\frac{s}{t}\end{equation}
\end{document}

Remarks:

\zref@extractdefault is expandable and can be directly used inside the value part of \setcounter.
\zref@refused complains, if the reference is not defined. LaTeX clears the warning status for undefined refernces in \begin{document}. Thus we need \AtBeginDocument (otherwise the final "LaTeX warning: There were undefined references." might be missing).

[Tex/LaTex] How to cross-reference several objects simultaneously

If you have three consecutive equations labeled, say, eq:1, eq:2, and eq:3 and need to cross-reference them jointly, you could do so with the help of the cleveref package and its \cref command:

\cref{eq:1,eq:2,eq:3}

cleveref is clever enough (pun intended) to sort and, if necessary, compress the numbers associated with the equations, generating the typeset output "eqs. (1) to (3)". If you don't want to see the word "equations" abbreviated to "eqs.", load the cleveref package with the noabbrev option.

The only real restriction if using the cleveref package is that you mustn't use a comma (,) as part of a label's name since -- as you can tell from the example above -- commas are used to separate labels from one another.

For much more on the topic of cross-referencing, including various aspects of the cleveref package, see the posting Cross-reference packages: which to use, which conflict? and the associated answers.

Best Answer

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