Binomial math symbol like but with square braces

Question

I want to generate symbol like this:

But I can't find a proper method in LyX. I tried frame decoration with \overset like this:

I also tried square braces with matrix inside, but is too large. It's surprising that LyX doesn't offer equal size room inside "frame decorations". How can I do here?

Answer 1

The construction you want to place is referred to under AMS math as a "small matrix". Here are the steps:

1. Insert > Math > Inline Formula

2. Insert > Math > Delimeters or click on the button and select the delimiters [ (for left) and ] (for right):

   

3. Within the inline formula type \smallmatrix and hit →. This inserts a smallmatrix environment inside the delimiters.

4. Now you insert elements in the same way you do a regular array or tabular; for example, insert x, then a new line (by pressing the button), moving over to the second element (by pressing TAB button), insert y.

   

5. Compile to view the output:

   

Answer 2

Try inserting the following TeX code.

\bigl[\!\begin{smallmatrix} a \\ b \end{smallmatrix}\!\bigr]

If that doesn't look right (it might be slightly too tall), you could also try

\left[\begin{smallmatrix} a \\ b \end{smallmatrix}\right]

or

\left[\!\begin{smallmatrix} a \\ b \end{smallmatrix}\!\right]

and see if they look nicer to you.