Using the xr package and - missing aux file? "Warning: No file SupplementalMaterial.aux LABELS NOT IMPORTED"

Question

I'm using Overleaf to write a paper with supplementary material and have been having substantial trouble with separating out Supplemental Material while maintaining references. My MWE is as follows - in Main.tex:

\documentclass{article}

\usepackage[hidelinks]{hyperref}
\usepackage{xr}
\externaldocument{SupplementalMaterial}

\begin{document}

This is text
\begin{equation} \label{eqn}
1+1=2
\end{equation}
Look at Eq.~(\ref{suppeqn})

\end{document}

In SupplementalMaterial.tex:

\documentclass{article}

\usepackage[hidelinks]{hyperref}

\usepackage{xr}

\externaldocument{Main}

\begin{document}

This is text
\begin{equation} \label{eqn}
1+1=2
\end{equation}
Look at Eq.~(\ref{suppeqn})

\end{document}

When I try to compile Main.tex, I get "Warning: No file SupplementalMaterial.aux LABELS NOT IMPORTED" and of course no references properly refer back to their labels. I've tried to find a solution online but frankly have not found an explanation as to where the aux file is supposed to come from, or what exactly it is. I think it's an auxiliary file used to hold information from the "first pass" for documents that take multiple iterations to compile.

How can I get my references to appear properly?

EDIT: Overleaf requires a special set-up to use xr, as detailed here. I was able to use the example linked in that page and port it to another documentclass for my specific uses without much trouble. I will leave this as an update, not an answer, because I do not yet understand why Overleaf requires this approach.

Answer 1

You forgot to create a file in the root folder of the project named "latexmkrc" and with content:

add_cus_dep( 'tex', 'aux', 0, 'makeexternaldocument' );
sub makeexternaldocument {
    if (!($root_filename eq $_[0]))
    {
        system( "latexmk -cd -pdf \"$_[0]\"" );
    }
}

This file is used at compile time to create the refer dependencies from the so created aux file. The problem you are experiencing is that at compile time the file aux cannot be found because it is not present, being the above instructions not present.

This should fix your issue, for further info see docs: https://www.overleaf.com/learn/how-to/Cross_referencing_with_the_xr_package_in_Overleaf

Answer 2

The set-up described by overleaf did not work for me. I had to make a relative file reference:

\myexternaldocument{./file_to_reference}