Want theorem and equation numbering linked under section

Question

Similar questions have been answered but the solutions didn't seem to give what I need.

I would like theorems and equations both to proceed as follows within each section.

Say we've begun Section 6. If I started with a theorem, it'd be Theorem 6.1. Then if I next used an equation, it would show up with numbering (6.2). The next equation or theorem (whichever is first) would be 6.3, and so on.

Answer 1

Using

\numberwithin{equation}{section}

(requires amsmath) makes the section counter to be used as prefix for equations, and then

\newtheorem{theorem}[equation]{Theorem}

makes theorem share the equation counter. A complete example

\documentclass{article}
\usepackage{amsthm}
\usepackage{amsmath}

\numberwithin{equation}{section}
\newtheorem{theorem}[equation]{Theorem}

\begin{document}

\section{Test section}
\begin{theorem}
test theorem
\end{theorem}

\begin{equation}
a=b
\end{equation}

\begin{theorem}
test theorem
\end{theorem}

\begin{align}
a &= b \\
&= c
\end{align}

\end{document}