Numbering theorems differently from the section's style

Question

I set the section numbering style to \Roman but now my theorems are numbered like so:

Theorem IV.2. instead of Theorem 4.2.

Is there a way of getting the latter but preserving the section numbering in Roman?

I've tried this Change theorem numbering from Roman to Arabic

\renewcommand\thetheorem{\arabic{section}.\arabic{theorem}}
\newtheorem{teo}{Theorem}[section]
\newtheorem{lemma}{Lemma}[section] 

I get

LaTex Error: \thetheorem command undefined.

Apparently this may work on other classes than article (in which I am)

Thanks in advance.

Answer 1

The problem is that you're trying to redefine \thetheorem before it's been defined. Move your \renewcommand after the \newtheorem command.

Also, the numbering for your theorems will follow the naming of the environments you've created so you should do, e.g.,

\renewcommand\theteo{\arabic{section}.\arabic{teo}}

and similarly for lemma.