Cubic Root

[Claus Hentschel]

How can I set a cubic root in math mode? Using \sqrt will produce a square
root. In Latex you can use \sqrt[3]{x+1} to define the cubix root of (x+1).
But wenn entering the '['-sign, LyX switches to show the root symbol like
you enter a space.

Thanks in advance
Regards
Claus