Library of reusable equations

Question

I want to do something that sounds simple. I want to create an external file where I will store my equations. Then I would like to be able to these equations and input them in a document without writing them all over again.

For example I would like something like that:

equations.tex

\begin{equation}
F = ma
\label{newton2ndLaw}
\end{equation}

\begin{equation}
det(A-\lambda I)=0
\label{eigValProb}
\end{equation}

And so on. Then I would go on and use the above file in a document.

document.tex

%other \usepackages ...
\usepackage{Magic package that does what I want}
\useeqlibrary{equations.tex} %I just made up this command

\begin{document}
Lorem Ipsum text

\begin{equation}
 \geteq{eigValProb} %made up this command also
 \label{anequationlabel}
\end{equation}

\end{document}

During compilation the equation TeX code would be inserted automatically in the document. Something like: "Type once, use many times, keep code clean". Does anyone of you have anything in mind?

Answer 1

The following uses some concepts from Is it possible to write all mathematical formulas in a separate file and add them in main file on-demand? to achieve your goal. It focuses on a redefinition of the equation environment to

1. capture the give \label; and
2. store the content of the equation environment in a macro \equation@<label>.

Retrieval is possible via \geteqn{<label>}. Since the stored content includes the original \label, there's technically no need to re-\label equations, but that doesn't really matter.

\documentclass{article}
\usepackage{filecontents,environ,etoolbox}% http://ctan.org/pkg/{filecontents,environ,etoolbox}
\robustify\label% To avoid premature expansion of \label when storing content

\let\oldequation\equation% Store \begin{equation}
\let\endoldequation\endequation% Store \end{equation}

\makeatletter
\def\eq@l@bel@gr@b#1\label#2#3\@nil{%
  \gdef\eq@gr@bbed@l@bel{#2}}% This macro extracts the \label inside \begin{equation}...\end{equation}
\providecommand{\env@equation@save@env}{}% To keep environ happy...
\providecommand{\env@equation@process}{}
\newcommand{\useeqlibrary}[1]{%
  \RenewEnviron{equation}{% Redefine equation environment to capture it's body
    \expandafter\eq@l@bel@gr@b\BODY\@nil% Extract \label
    \expandafter\protected@xdef\csname equation@\eq@gr@bbed@l@bel\endcsname{\BODY}% Store equation in macro
  }%
  \input{#1}% Parse input file
}

\AtBeginDocument{% Restore original equation environment
  \let\equation\oldequation% Restore \begin{equation}
  \let\endequation\endoldequation% Restore \end{equation}
}

\newcommand{\geteqn}[1]{\csname equation@#1\endcsname}% Extract macro based on label
\makeatother

\begin{filecontents*}{equations.tex}
\begin{equation}
  F = ma \label{newton2ndLaw}
\end{equation}

\begin{equation}
  \mathrm{det}(A - \lambda I) = 0 \label{eigValProb}
\end{equation}
\end{filecontents*}

\useeqlibrary{equations.tex} %I just made up this command

\begin{document}

\begin{equation}
  \geteqn{eigValProb} %made up this command also
  \label{anequationlabel}
\end{equation}

See~(\ref{eigValProb}) or~(\ref{anequationlabel}).

\end{document}

As mentioned, this only works with the equation environment. Could possibly be extended to other environments in a similar manner. Moreover, it extracts only the first \label (if more than one exist).