You can't use `\spacefactor' in math mode

Question

Yesterday, I installed TexLive2017 to Linux Mint to use lualatex, and lualatex 1.0.4 is installed now. But so simple a code shown below doesn't work properly.

\documentclass{ltjarticle}
\usepackage{luatexja}
% \documentclass{article}

\begin{document}

\begin{eqnarray}
f(x) = x^2 + 4 \label{eq:1}
\end{eqnarray}

then we can find
\begin{eqnarray}
a = 4.\ \ \ (\mathrm{by\ eq.(\ref{eq:1})})
\end{eqnarray}

\end{document}

The error message is this.

*! You can't use `\spacefactor' in math mode.
\@setref ...d}\else \expandafter #2#1\spacefactor 
                                                  \@m {}\fi 
l.13 a = 4.\ \ \ (\mathrm{by\ eq.(\ref{eq:1})}
                                            )
? 
Missing character: There is no è (U+00E8) in font cmex10!
[1*

The code can be complied without any errors if I write the code from scratch and save as new name and compile for the first time. It means the error occurs when \label{eq:1} and \ref{eq:1} are tethered.

How can I solve this problem? This code works properly on my mac (texlive2016, lualtex 0.95.0). If I comment out the first two lines and uncomment the third line, the problem dissolves at once, but I have to use luatex Japanese packages as I'm Japanese.

Answer 1

The issue you've uncovered isn't related directly to Lua(La)TeX. Instead, it would appear to be related to the insertion of whitespace inside a \mathrm{...} wrapper. I suggest that you load the amsmath package and use the \text{...} wrapper instead.

Oh, and please do not use the badly-deprecated eqnarray environment. Instead, use the equation environment for single-line equations and the align environment (provided by the amsmath package) for multi-line equations.

\documentclass{ltjarticle}
\usepackage{amsmath} % for '\text' macro
\usepackage{luatexja}
\usepackage{cleveref} % optional, for '\cref' macro

\begin{document}    
\begin{equation} \label{eq:1}
f(x) = x^2 + 4 
\end{equation}
then we can find
\begin{equation}
a = 4\quad(\text{by \cref{eq:1}})
\end{equation}  
\end{document}