You should not try to use a fractional, but keep the theorem counter as integer, and precede this counter with another value., like:
Code:
\newcounter{bar}
\newtheorem{law}{Law}[bar]
\begin{law}
It always seems to go wrong
\end{law}
\addtocounter{bar}{2}
\begin{law}
This one is right
\end{law}
Which produces:
Law 0.1 It always seems to go wrong
Law 2.2 This one is right
Alternatively, you can include the chapter or section number as part of you theorem counter:
Code:
\newtheorem{law}{Law}[chapter]
starts with Law 1.1.
See this page:
http://www.personal.ceu.hu/tex/counters.htm on counters.
jlinkels