Numbering theorem-like environments

amsthm provides \swapnumbers which reverses the display of Theorem <num>. to <num> Theorem. Here's a minimal example:

\documentclass{article}
\usepackage{amsthm}% http://ctan.org/pkg/amsthm
\swapnumbers % Switch number/label style
\theoremstyle{plain}
\newtheorem{theorem}{Theorem}
\newtheorem{example}[theorem]{Example}
\begin{document}
\begin{theorem}
This is a theorem.
\end{theorem}
\begin{example}
This is an example.
\end{example}
\end{document}​

In the above example, the example environment shares the theorem counter. Moreover, both are defined with the plain style. There are others available, as shown in the amsthm package documentation.

amsthm has a command \swapnumbers that, when specified before the \theoremstyle or \newtheorem elenments for which numbers are to come first, will do what you want.

for details, see the amsthm user's guide, section 4.2 on "number swapping".

There are few ways to create theorem like environments. Here's an option using ntheorem

The trick is to make a new theoremstyle using the arguments

• ##1 which is the name of the theorem
• ##2 which is the number of the theorem
• ##3 which is the optional title of the theorem

Using the examples in the ntheorem documentation as a guide, you can tweak this to suit your needs.

\documentclass{article}

\usepackage{lipsum}
\usepackage{ntheorem}

\makeatletter
\newtheoremstyle{numberfirst}%
{\item[\theorem@headerfont{##2\theorem@separator\hskip\labelsep ##1}]}%
{\item[\theorem@headerfont{##2\theorem@separator\hskip\labelsep ##1} (##3)]}%
\makeatother

\theoremstyle{numberfirst}
\theoremseparator{.}
\newtheorem{mytheorem}{Theorem}

\begin{document}

\begin{mytheorem}
\lipsum[1]
\end{mytheorem}

\begin{mytheorem}[Optional title]
\lipsum[1]
\end{mytheorem}

\end{document}