copy and paste this into LyX, then mark it with the cursor and then ctrl-m, which should convert it into math. Maybe that this causes and error, when LyX tries to convert it in the right way. Then insert it by hand.
\[ |x| \underset{\makebox[0pt]{$\substack{\uparrow\\(x>0)}$}}{=} x \]
Thanks a lot! Could not one have a longer arrow?
Paul
