cref and external documents

Question

I want two things: 1. to be able to use \cref across different files and 2. to display certain environments (just the theorems for example) from an external file.

What I mean by (2) is the following. Suppose I have theorems in a file called external.tex. Suppose I'm working in main.tex. I want all of the theorems from external.tex to be pasted into main.tex without having to rewrite them.

Here is my code for (1):

external.tex:

\documentclass{article}
\usepackage{amsmath,cleveref}
\newtheorem{lem}{Lemma}
\begin{document}
\begin{lem}\label{mylemma}
blah blah blah
\end{lem}
\end{document}

main.tex:

\documentclass{article}
\usepackage{cleveref}
\usepackage{xr}
\externaldocument[ext:]{external}
\begin{document}
using \cref{ext:mylemma}
\end{document}

When I use \ref{ext:mylemma}, it works. It just displays the number 1. When I use \cref{ext:mylemma}, I get "?? 1". How do I fix this?

Answer 1

cleveref in main.tex does not know what the label type lem is (as indicated by the warning: cref reference format for label type `lem' undefined). Therefore, add \newtheorem{lem}{Lemma} to the preamble of main.tex to define it therein.

\documentclass{article}
\usepackage{cleveref}
\newtheorem{lem}{Lemma}
\usepackage{xr}
\externaldocument[ext:]{external}
\begin{document}
using \cref{ext:mylemma}
\end{document}

Answer 2

Instead of repeating the definition of newtheorem lemma it's possible to make cleveref aware of lem using \crefname or \Crefname

\documentclass{article}

\usepackage{xr}
\usepackage{cleveref}
\externaldocument[ext:]{external}
\crefname{lem}{lemma}{lemmata}
\Crefname{lem}{Lemma}{Lemmata}

\begin{document}
using \cref{ext:mylemma} or \Cref{ext:mylemma}
\end{document}