Cross-Referencing Equations Before Definition In Latex

Definition of Cross-referencing Equations

In LaTeX documents containing mathematical equations, authors may need to reference an equation before its official definition in the source code. This is known as forward referencing or cross-referencing equations before definition.

For example, an article may mention "Equation 12" on page 1 before Equation 12 is typeset on page 5. Without special handling, LaTeX would output an undefined reference error. Enabling forward references allows the author to mention equations out of order without disrupting the document compilation.

Why Cross-referencing Undefined Equations Causes Errors

By default, LaTeX requires equations to be defined before they are referenced. The LaTeX compiler keeps track of equation numbers sequentially as it processes the source text. When LaTeX encounters a reference for an equation that has not yet been defined, it has no number to output for that reference, causing a compiler error.

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This strict ordering requirement conflicts with the writing needs of authors in many cases. Academics may want to discuss key results by equation number in the introduction before defining those equations in detail. Referencing an upcoming derivation by equation number can also help orient readers and foreground important conclusions.

Using the eqref Package to Allow Forward References

The eqref package for LaTeX allows authors to circumvent the default equation ordering constraints. Developed by Takayuki Yato and available on CTAN, eqref introduces a new \eqref command and counter system to enable forward references.

With eqref, authors can write \eqref{myequation} to refer to "Equation myequation" before that label is officially defined. The compiler will accept the forward reference and output a placeholder string where needed in the final document.

Importing and Enabling the eqref Package

To use the eqref package for forward equation references, the package must first be imported in the preamble using \usepackage. For example:

\usepackage{eqref}

Next, the package functionalities must be activated by including the \eqrefpackage command after the \begin{document} directive:

\begin{document}
\eqrefpackage{true}
...

With eqref activated, authors may then write cross-references for equation labels before the corresponding \label command.

Referencing Yet-to-Be Defined Equations

The core functionality enabled by eqref is the ability to reference equations before they are technically defined with \label in the LaTeX source code.

For example, immediately after importing and activating eqref, an author could write:

As we saw in Equation \eqref{keyresult}, our methodology achieves a 10\% reduction in errors.

In this case, there is not yet any defined equation with the keyresult label. But eqref allows LaTeX to compile the reference anyway, filling in the appropriate equation number later.

Example LaTeX Code with eqref

Here is a full LaTeX source code example using the eqref package to cross-reference equations before definition:

\documentclass{article}
\usepackage{eqref}

\begin{document}

\eqrefpackage{true} 

In Equation \eqref{myequation}, we demonstrate the efficacy of our new method.

\begin{equation}\label{myequation}
   a^2 + b^2 = c^2
\end{equation}

As shown in Equation \eqref{myequation}, the Pythagorean theorem...

\end{document}

In this case, the reference on line 7 is allowed to precede the actual equation definition on line 10. The document will compile without errors, with \eqref outputting the correct equation number on second reference.

Limitations of the eqref Package

The eqref package enables equation forward referencing functionality but does have some limitations:

Equation numbers may change upon compilation if new equations are added before existing labels, breaking previously valid references
Callouts within multiline equations may fail or produce inconsistent references
Compatibility issues may arise in some edge case scenario with other equation-related packages
Recompilation is required for number updates after new equations are inserted

Accordingly, eqref should primarily be used for convenience of writing, not as a solution for articles with strict publishing requirements. Authors should be prepared to revisit references upon major content changes when using eqref.

Other Methods for Forward Referencing

In addition to the eqref package, there are some other options for cross-referencing equations in LaTeX before they are defined:

Manually specifying numbers: Authors can explicitly write out numbers like "Equation 12" and adjust later as needed rather than relying on a generated counter.
Dummy text insertion: Insert a dummy \label command with placeholder text, then reference this before replacing with the actual equation.
Draft labels: Use the xxx label for references, then do a find/replace on the entire document after equation numbering is finalized.

However, these methods can become tedious or unreliable for longer documents. The eqref package provides the best automated system for equation forward references in LaTaX.

Additional Packages for Advanced Equation Handling

Beyond eqref, LaTeX offers additional packages for specialized equation environments and customizations:

amsmath: Expanded math environments like matrices, equation arrays, and multi-line equations.
IEEEeqnarray: For numbered equation arrays required by IEEE journals.
empheq: Module with emphasis and box options for equations.
ntheorem: Flexible theorem and proof environments linking equations.

These complementary packages build on the foundations of eqref and LaTeX's robust typesetting of mathematical symbols. Authors with advanced publishing needs should explore available packages for equations in formats required by target journals or publishers.

When to Avoid Forward Referencing Equations

While the eqref package enables convenient forward referencing in many cases, directly referencing upcoming equations can also create reader confusion in some scenarios. Authors should avoid future references when:

The justification derives mainly from the equation itself - don't forward reference derivations or logical proofs.
Equations are grouped in rapid succession - use trailing references instead for clarity.
There are already many forward references - re-order to reduce dependency on eqref.
Intended for publication without further chance to update - numbering changes could invalidate references.

In such cases, directly preceding or subsequent references generally make relations clearer rather than referencing still-unseen equations. Authors should judge when forward references improve flow versus feeling disjointed from content.

Best Practices for Equation Definition and References

To leverage eqref efficiently and minimize reader confusion from out-of-order equations, authors should:

Maintain consistency in logical ordering aside from forward references.
Recompile frequently when editing to catch invalid references early.
Use distinguishing terminology like Theorem 1 or Equation 12 rather than just standalone numbers.
Break documents into equation environments rather than relying on one master counter.
Proofread references carefully upon finalizing manuscripts to match updated numbering.

Following eqref best practices helps ensure authors fully gain the writing convenience of equation forward referencing while producing clear, orderly documents.