I'm a LyX beginner so please don't worry about stating the obvious.

I'm numbering equations by section in my document. That's working fine. However, when I refer to the same equation later on in the text I can't see how I can give it its original reference number.

So if `y=x+2`

is numbered as `4.2.2`

, I want to be able to give `y=x+2`

the same number next time I use it. Is there a straightforward way of doing this? I'm assuming I have to sandwich the equations in code of some sort.

I'm using the `scrbook`

(Koma-script) document class. I'm using the modules: Theorems (AMS), Theorems Numbered by Chapter, Number Equations by Section.

The LaTeX preamble is:

```
\usepackage{microtype}
\usepackage {hyperref}
\usepackage{amsmath}
\numberwithin{equation}{section}
\setkomafont{captionlabel}{\bfseries}
```

To date I've been changing the code in my LyX documents using Windows Notepad. Is that the best way of editing code?

## Best Answer

Werner provided the real method, here's translated to LyX GUI (I tested this only with the simplest example):

For the first instance of the equation, add a label by right clicking the equation and selecting "Equation label".

The second time, write the equation, and while

still in math modeadd a TeX code box, by pressingCtrl+L.In this code box, write

`tag{`

(the backslash should already be there, at least it was when I tested). This should give you a set of braces with a small box in it.Choose Insert --> Cross-reference, and select the reference to the first instance of the equation.

A video showing the process, the TeX code menu option was outside the frame: http://www.youtube.com/watch?v=NTqLMO8WS6w