ok, I see. A problem with LyX. Write into the preamble:
\newtheorem{theorem}{FrenchTheorem} \let\thm\theorem
It works almost perfectly. Without inserting "\let\thm\theorem" in the preamble, the output DVI file displays "Theorem 0.1" whereas otherwise it appears "FrenchTheorem 1.". It seems that anything was changed in the theorem numbering.
change the first line to
\newtheorem{theorem}{FrenchTheorem}[section]
Herbert,
After the changes suggested by you, I get the following sequence in the DVI file:
Theorem 0.1. This is the first theorem.
Lemma 0.2. This the only lemma.
Theorem 0.2. This is the second theorem.
Should not the last theorem be labeled as “Theorem 0.3”?
Thank you in advance!
Paul
