Recently I plan to transplant one of my old post from other site to this new site (you can find the new post here). The old post contains quite a lot of equations in LaTeX format. With so much equations in one post, I find it really hard to refer to some equations without using proper numbering. So I decide to figure out how to use LaTeX equation numbering properly in my post.

# Enable MathJax and equation numbering

Update 2018-05-16: This feature has been incorporated into NexT and released in version 6.3.0. You can skip this section safely now.

If you use NexT theme, see here on how to enable MathJax. By default, equation numbering is disable by Mathjax, you can enable equation numbering in two ways.

## First way

According to MathJax official documentation, just paste the following script into the front of your Markdown file,

<script type="text/x-mathjax-config">
MathJax.Hub.Config({
TeX: { equationNumbers: { autoNumber: "AMS" } }
});
</script>

## Second way

Find the file mathjax.swig under theme NexT root (it is in the folder layout\_third-party\). Change the following snippet

<script type="text/x-mathjax-config">
MathJax.Hub.Config({
tex2jax: {
inlineMath: [ ['$','$'], ["\$","\$"]  ],
processEscapes: true,
skipTags: ['script', 'noscript', 'style', 'textarea', 'pre', 'code']
}
});
</script>

to

<script type="text/x-mathjax-config">
MathJax.Hub.Config({
tex2jax: {
inlineMath: [ ['$','$'], ["\$","\$"]  ],
processEscapes: true,
skipTags: ['script', 'noscript', 'style', 'textarea', 'pre', 'code']
},
TeX: {equationNumbers: { autoNumber: "AMS" }}
});
</script>

Thanks to the post here.

# Make the automatic equation numbering work

In general, to make the automatic equation numbering work, you have to wrap your LaTeX equations in equation environment. Using the plain old style (i.e., wrap an equation with two dollar signs in each side) will not work. How to refer to an equation? Just give a \label{} tag and then in your later text, use \ref{} or \eqref{} to refer it. Using \eqref{} is preferred since if you use \ref{}, there are no parentheses around the equation number. But there are some variations I would like to elaborate.

## Simple equation

For simple equations like the following,

$$$E=mc^2$$\label{eq1}$

It is easy to write and cite. Just use $\eqref{eq1}$ to refer to equation $\eqref{eq1}$. The code to produce the equation $\eqref{eq1}$ is

$$\begin{equation} E=mc^2 \end{equation}\label{eq1}$$

## Multi-line equation

For multi-line equations, inside the equation environment, you can use the aligned environment to split it into multiple lines. For example,

\begin{aligned} a &= b + c \\ &= d + e + f + g \\ &= h + i \end{aligned}\label{eq2}

Equation $\eqref{eq2}$ is a multi-line equation. The code to produce equation $\eqref{eq2}$ is

\begin{equation} \begin{aligned} a &= b + c \\ &= d + e + f + g \\ &= h + i \end{aligned} \end{equation}\label{eq2}

## Multiple aligned equations

We can use align environment to align multiple equations. Each of these equations will get its own numbers.

\begin{align} a &= b + c \label{eq3} \\ x &= yz \label{eq4}\\ l &= m - n \label{eq5} \end{align}

There are three aligned equations above: equation $\eqref{eq3}$, equation $\eqref{eq4}$ and equation $\eqref{eq5}$.

Use the following code to reproduce,

\begin{align} a &= b + c \label{eq3} \\ x &= yz \label{eq4}\\ l &= m - n \label{eq5} \end{align}

Since align in and of itself is a complete equation environment (read here about the difference between aligned and align in LaTeX). You do not need to wrap it with equation environment.

### What if you do not want to number some equations

In the align environment, if you do not want to number one or some equations, just use \nonumber right behind these equations. Like the following.

\begin{align} -4 + 5x &= 2+y \nonumber \\ w+2 &= -1+w \\ ab &= cb \end{align}

The code to reproduce is

\begin{align} -4 + 5x &= 2+y \nonumber \\ w+2 &= -1+w \\ ab &= cb \end{align}

# Use \tag to tag equations

Sometimes, you want to use more "exotic" style to refer your equation. You can use \tag{} to achieve this. For example

$x+1\over\sqrt{1-x^2} \tag{i}\label{eq_tag}$

Equation $\eqref{eq_tag}$ use \tag{} instead of automatic numbering. The code to produce it is

$$x+1\over\sqrt{1-x^2} \tag{i}\label{eq_tag}$$