How to Make Theorems and Equations Share Numbering

Question

How do I make theorems and equations share numbering? The default numbering system goes like this:

Theorem 1. Blah blah blah...

1=0+1 (1)

Theorem 2. More blahs...

Theorem 3. No more blahs...

2=1+1 (2)

The numbers in the parenthesis are equation numbers. How do you change the above into this:

Theorem 1. Blah blah blah...

1=0+1 (2)

Theorem 3. More blahs...

Theorem 4. No more blahs...

2=1+1 (5)

I am using ntheorem package, but couldn't figure out how. Would I need to use a different package?

Answer 1

You just need to tell it to number your theorems using the equation counter.

\newtheorem{thm}[equation]{Theorem}

Now \begin{thm} will be numbered the same as \begin{equation}.

Answer 2

See the amsthm documentation. It is only a few pages long and includes a discussion of this point (as well as lots of others relating to theorem styles).

Answer 3

Here's a solution using the thmtools package in addition to ntheorem.

\documentclass{article}

\usepackage{ntheorem}
\usepackage{thmtools}
\declaretheorem[numberlike=equation]{theorem}

\begin{document}

\begin{equation}
a^2 + b^2 = c^2
\end{equation}

\begin{theorem}
Some text.
\end{theorem}

\end{document}