breqn and lineno incompatibility

Question

Journals often require line numbering for the review process. This is achieved using lineno package. Next, it seems like a good idea to use breqn package (dmath environment) to auto-break long formulas.

But, not switching line numbering off before dmath environment (\begin{nolinenumbers}, \end{nolinenumbers}), causes pdftex to hang (it just consumes 100% CPU, and compilation never finishes).

1. Why is this happening?

2. How to redefine dmath (and dmath*) environment to have nolinenumbers environment automaticly added around it.

   \documentclass[twocolumn,12pt]{article}
   \usepackage{breqn}
   \usepackage{lineno}
   
   \begin{document}
   \linenumbers
   
   First paragraph.
   
   \begin{nolinenumbers}
   \begin{dmath}
   f(x)=x^9+x^8-x^7+x^6-x^5-x^4-x^3+x^2-x+1.
   \end{dmath}
   \end{nolinenumbers}
   \end{document}  
   

Answer 1

Load etoolbox before breqn:

\documentclass[twocolumn,12pt]{article}

\usepackage{etoolbox}
\usepackage{breqn}
\usepackage{lineno}

\BeforeBeginEnvironment{dmath}{\begin{nolinenumbers}}%
\AfterEndEnvironment{dmath}{\end{nolinenumbers}}


\begin{document}
\linenumbers

First paragraph.

%\begin{nolinenumbers}
\begin{dmath}
f(x)=x^9+x^8-x^7+x^6-x^5-x^4-x^3+x^2-x+1.
\end{dmath}
%\end{nolinenumbers}

Second Paragraph
\end{document}  

Answer 2

The lineno manual states that it "does not work well with display math" (p.13). One way is to wrap equations in in the linenomath environment for common math environments, but this does not work for dmath. So we may use etoolbox to patch dmath to exclude linenumbers automatically (and equivalently for dmath*).

\documentclass[twocolumn,12pt]{article}

\usepackage{etoolbox}

\usepackage{breqn}
\usepackage{lineno}

\BeforeBeginEnvironment{dmath}{\begin{nolinenumbers}}
\AfterEndEnvironment{dmath}{\end{nolinenumbers}}

\BeforeBeginEnvironment{dmath*}{\begin{nolinenumbers}}
\AfterEndEnvironment{dmath*}{\end{nolinenumbers}}


\begin{document}
\linenumbers

First paragraph.

\begin{dmath}
f(x)=x^9+x^8-x^7+x^6-x^5-x^4-x^3+x^2-x+1.
\end{dmath}

Second paragraph

\begin{dmath*}
f(x)=x^9+x^8-x^7+x^6-x^5-x^4-x^3+x^2-x+1.
\end{dmath*}

\end{document}

Loading etoolbox after breqn and carefully reading the error message reveals that etoolbox fails to patch \end and hence the \AfterEndEnvironment will not work. There appears to be an incompatibility (probably breqn patches something as well). So simply load etoolbox before.