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# Math and equations
Jupyter Book uses [MathJax](http://docs.mathjax.org/) for typesetting math in your HTML book build.
This allows you to have LaTeX-style mathematics in your online content.
This page shows you a few ways to control this.
:::{seealso}
For more information about equation numbering,
see the [MathJax equation numbering documentation](http://docs.mathjax.org/en/v2.7-latest/tex.html#automatic-equation-numbering).
:::
:::{tip}
By default MathJax version 2 is currently used.
If you are using a lot of math, you may want to try using version 3, which claims to improve load speeds by 60 - 80%:
```yaml
sphinx:
config:
mathjax_path: https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js
```
See the [Sphinx documentation](sphinx:sphinx.ext.mathjax) for details.
:::
## In-line math
To insert in-line math use the `$` symbol within a Markdown cell.
For example, the text `$this_{is}^{inline}$` will produce: $this_{is}^{inline}$.
+++
## Math blocks
You can also include math blocks for separate equations. This allows you to focus attention
on more complex or longer equations, as well as link to them in your pages. To use a block
equation, wrap the equation in either `$$` or `\begin` statements.
For example,
```latex
$$
\int_0^\infty \frac{x^3}{e^x-1}\,dx = \frac{\pi^4}{15}
$$
```
results in:
$$
\int_0^\infty \frac{x^3}{e^x-1}\,dx = \frac{\pi^4}{15}
$$
+++
or, by adding to your `_config.yml`:
```yaml
parse:
myst_extended_syntax: true
```
you can use:
```latex
\begin{gather*}
a_1=b_1+c_1\\
a_2=b_2+c_2-d_2+e_2
\end{gather*}
\begin{align}
a_{11}& =b_{11}&
a_{12}& =b_{12}\\
a_{21}& =b_{21}&
a_{22}& =b_{22}+c_{22}
\end{align}
```
which results in:
\begin{gather*}
a_1=b_1+c_1\\
a_2=b_2+c_2-d_2+e_2
\end{gather*}
\begin{align}
a_{11}& =b_{11}&
a_{12}& =b_{12}\\
a_{21}& =b_{21}&
a_{22}& =b_{22}+c_{22}
\end{align}
:::{seealso}
The MyST guides to [dollar math syntax](myst-parser:syntax/math), [LaTeX math syntax](myst-parser:syntax/amsmath), and [how MyST-Parser works with MathJax](myst-parser:syntax/mathjax).
For advanced use, also see how to [define MathJax TeX Macros](sphinx/tex-macros).
:::
+++
### Numbering equations
If you'd like to number equations so that you can refer to them later, use the **math directive**.
It looks like this:
````md
```{math}
:label: my_label
my_math
```
````
For example, the following code:
````md
```{math}
:label: my_label
w_{t+1} = (1 + r_{t+1}) s(w_t) + y_{t+1}
```
````
will generate
```{math}
:label: my_label
w_{t+1} = (1 + r_{t+1}) s(w_t) + y_{t+1}
```
Alternatively you can use the dollar math syntax with a prefixed label:
```md
$$
w_{t+1} = (1 + r_{t+1}) s(w_t) + y_{t+1}
$$ (my_other_label)
```
which generates
$$
w_{t+1} = (1 + r_{t+1}) s(w_t) + y_{t+1}
$$ (my_other_label)
:::{note}
Labels cannot start with an integer, or they won't be able to be referenced and
will throw a warning message if referenced. For example, `:label: 1` and `:label: 1eq` cannot
be referenced.
:::
### Linking to equations
If you have created an equation with a label, you can link to it from within your text
(and across pages!).
You can refer to the equation using the label that you've provided by using
the `{eq}` role. For example:
```md
- A link to an equation directive: {eq}`my_label`
- A link to a dollar math block: {eq}`my_other_label`
```
results in
- A link to an equation directive: {eq}`my_label`
- A link to a dollar math block: {eq}`my_other_label`
:::{note}
`\labels` inside LaTeX environment are not currently identified, and so cannot be referenced.
We hope to implement this in a future update (see [executablebooks/MyST-Parser#202](https://github.com/executablebooks/MyST-Parser/issues/202))!
:::