I'll try to explain it:
I've just started writing a new document in Lyx, an article document, and the first thing I would like to do is to write a "header".
So I select "Right header" and then I write what I want. Just after that I press the "eyed-button" to see it as a PDF but the next message appears:

"Undefined control sequence.
\rhead
{lolo}
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined."
I definitly don't know how to fix it.
Thank for your comments
