\newtheorem{theorem}{FrenchTheorem}
into the preamble. I'd forgotten that in amsart the theorem isn't defined.
Herbert, I did as you suggested and the file was perfectly compiled. However, the label "Theorem" remains on the DVI file. (FrenchTheorem label does not show up.) I am using LyX 1.3.2.
hm, curious. please insert into the preamble as first line \listfiles and send me the logfile and the temporary tex file, which are in /tmp/lyx..../...
Herbert
