Wrong numeration using \newtheorem

Question

by using the \newtheorem, the latex does not number the theorems in the right order.

used code is

\newtheorem{theo.a}{Proposition}
\begin{theo.a}
this is the proposition.
\end{theo.a}

changing into

\newtheorem{theo.a}{Proposition}[2]
\begin{theo.a}
this is the proposition.
\end{theo.a}

leads to a wrong output as well (Proposition 1 is replaced by Proposition .1)

Answer 1

Please always post complete small documents, not unusable fragments however like most latex counter constructs,

\newtheorem{theo.a}{Proposition}

defines theo.a to increment the counter before printing it so if you want to start at Proposition 42 then you just need

\setcounter{theo.a}{41}
\begin{theo.a}
this is the proposition.
\end{theo.a}