I wanted to change the math stuff in my book from the italic like to the text-like appearance. I marked the math stuff and clicked on the math icon in the Lyx Icon bar. This turns the italic math to text-like appearance
-but apparently it is not math anymore.
So how do I do it in a proper way?
Either you mark the characters, open the math panel with a right click. Here you choose the font 'Roman' from the math-font menu.
Or you insert the command \mathrm in a formula and copy the characters in the appearing pink box.
As you are a german, you'll find this all described here:
http://wiki.lyx.org/pmwiki.php/LyX/LyXMathebefehle
(chapter 11)
regards Uwe