MY PROCEDURE FOR DUMMIES WAS WRONG, SORRY
The problem comes from the fact that within the box generated with the
\fbox command you are back to the text environment, hence to type
equations, once you are inside the box, you have to insert an online
formula
HOW TO PUT A BOX AROUND AN EQUATION
Resume for dummies n. 2, hopefully the right one:
1) go to math mode with the command INSERT --> MATH --> DISPLAY FORMULA
2) type \fbox (visible box) and then a press carriage return or the
spacebar
3) the text "\fbox" disappear and a framed box appears
4) put the cursor inside the box (WARNING, YOU ARE IN TEXT MODE NOW!!)
5) go again to math mode with the command INSERT --> MATH --> INLINE
FORMULA
6) type the equation there inside
7) eventually apply equation number as usual
As a comment,
1) the box reduces the height of the equation, I don't know why, but
this seems to be a feature of Latex, not of Lyx
2) if you need the eqnarray environment within the box you are in
troubles, it seems it does not work. A workaround is given by inserting
a matrix from the math panel and then aligning the contents with
EDIT-->MATH-->ALIGNEMENT--> WHAT YOU NEED
Hope this is useful, thanks to all
Giorgio Zavarise
