[Tex/LaTex] How to add two vertical lines in LyX

Question

I need to do two vertical lines in LyX like this:

How to do that?

Answer 1

To have an equivalent of the LaTeX \left\| \begin{array}{l} \mbox{For } a \\ f(a') \\ \end{array} \right.

First, you go I̲nsert>Math̲>Delimiter̲s

You uncheck the box "Keep matched" and you chose the correct left and right:

Then, inside these delimiters, you will be in maths mode and have access to the maths toolbar. Add a matrix 2x1:

Finally, you may want to use a \mbox for your "For ", Lyx should auto-complete.

Fill the matrix with the wanted elements, and you get:

You could also use \textit instead of the \mbox and have: