# Automatic Equation Numbering

The TeX input processing in MathJax can be configured to add equation numbers to displayed equations automatically. This functionality is turned off by default, but it is easy to configure MathJax to produce automatic equation numbers by adding:

    window.MathJax = {
      tex: {
        tags: 'ams'
      }
    };

to tell the TeX input processor to use the AMS numbering rules (where only certain environments produce numbered equations, as they would be in LaTeX). It is also possible to set the tagging to `'all'`, so that every displayed equation will get a number, regardless of the environment used.

You can use `\notag` or `\nonumber` to prevent individual equations from being numbered, and `\tag{}` can be used to override the usual equation number with your own symbol instead (or to add an equation tag even when automatic numbering is off).

Note that the AMS environments come in two forms: starred and unstarred. The unstarred versions produce equation numbers (when `tags` is set to `'ams'`) and the starred ones don’t. For example

    \begin{equation}
       E = mc^2
    \end{equation}

will be numbered, while

    \begin{equation*}
       e^{\pi i} + 1 = 0
    \end{equation*}

will not be numbered (when `tags` is `'ams'`).

You can use `\label` to give an equation an identifier that you can use to refer to it later, and then use `\ref` or `\eqref` within your document to insert the actual equation number at that location, as a reference. For example,

    In equation \eqref{eq:sample}, we find the value of an
    interesting integral:

    \begin{equation}
      \int_0^\infty \frac{x^3}{e^x-1}\,dx = \frac{\pi^4}{15}
      \label{eq:sample}
    \end{equation}

includes a labeled equation and a reference to that equation. Note that references can come before the corresponding formula as well as after them.

You can configure the way that numbers are displayed and how the references to them by including the `tagformat` extension, and setting options within the `tagformat` block of your `tex` configuration. See the tagformat extension for more details.

If you are using automatic equation numbering and modifying the page dynamically, you can run into problems due to duplicate labels. See Resetting Automatic Equation Numbering for how to address this.