Why doesn't lineno number a paragraph when it is followed by an indented equation?

Question

While searching for the source of missing line numbers, I realized that paragraphs followed immediately by an equation have no line number, but this is fixed when a space is included,

e.g. the following document has no line numbers:

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{lineno}
\linenumbers
\begin{document}
Lorem ipsum dolor sit amet, consectetur adipiscing elit: 
$$1+1=2$$ 
\end{document}

But this one does:

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{lineno}
\linenumbers
\begin{document}
Lorem ipsum dolor sit amet, consectetur adipiscing elit: 

$$1+1=2$$ 
\end{document}

The only difference is the space above the equation.

• Why is this the case (and is there a simple workaround)?

• Is it incorrect to include an indented equation in a paragraph, or is this just a 'feature' of lineno?

Answer 1

$$...$$ is obsolete, see Why is \[ ... \] preferable to $$ ... $$?. If you use the correct LaTeX displayed math environment, the numbering works without the empty line:

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{lineno}
\linenumbers
\begin{document}
Lorem ipsum dolor sit amet, consectetur adipiscing elit: 
\[1+1=2\] 
\end{document}

Answer 2

For line numbering to be done correctly the math environments has to be wrapped using the \begin{linenomath*} and \end{linenomath*} code:

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{lineno}
\linenumbers
\begin{document}
For line numbering to be done correctly the math environments has to be wrapped using the ''linenomath`` code as follows: 

\begin{linenomath*}
    \begin{equation}
        a^2=b^2+c^2
    \end{equation}
\end{linenomath*}
some text here some text here  some text here some text here some text here some text here some text here some text here some text here some text here some text here some text here some text here some text here some text heresome text here. 
\end{document}

Answer 3

Mathematics environments need to be wrapped by \begin{linenomath*} and \end{linenomath*} as mentioned in Bamzi's answer.

A quick way to change all your equations is to renew the equation environment by adding the code

\let\oldequation\equation
\let\oldendequation\endequation

\renewenvironment{equation}
  {\linenomathNonumbers\oldequation}
  {\oldendequation\endlinenomath}

before \begin{document}.

Answer 4

As @Denis has said, the mathlines option can be used in some cases (for article, but not for iopart, for example). If you do so, and you are also using the amsmath package, be sure to load lineno after amsmath. Sometimes other packages load amsmath, be aware of that; you can check the log file to see this. This MWE can be used to easily see the effect of that change:

\documentclass{article}
% \usepackage[mathlines]{lineno}
\usepackage{amsmath}
\usepackage[mathlines]{lineno}

\begin{document}
    \linenumbers

    Line number

    No line number if lineno is loaded before amsmath!
    \begin{equation}
        x = y
    \end{equation}

    Line number
\end{document}

Answer 5

Use the mathlines option for lineno and write:

\begin{linenomath}
\begin{align*}
z_{1}&=x_{1}+y_{1}\\
z_{2}&=x_{2}+y_{2}
\end{align*}
\end{linenomath}

to obtain

Answer 6

The answer from Stefan Pinnow fixed the equation command. Use this to fix the align command in the same style:

\let\oldalign\align
\let\oldendalign\endalign

\renewenvironment{align}
  {\linenomathNonumbers\oldalign}
  {\oldendalign\endlinenomath}