Equation numbering: how can I get (1.1), (2.1), (3.2) in the article class?

Question

I know how to make them do 1, 2, 3....just need to know 1.1, 1.2, 3.2, 3.3 etc

\begin{align}

\end{align}

This will produce 1, 2, 3 etc, I know this but need the other.

Answer 1

The question can also be interpreted that the first number of the composite equation number should be the section number. The other two classes report and book are using composite equation numbers, whose first part is the chapter number. The article class does not have chapters and uses plain equation numbers.

Package amsmath (this is used because of align) provides an easy way to add the section number:

\numberwithin{equation}{section}

Complete example:

\documentclass{article}
\usepackage{amsmath}

\numberwithin{equation}{section}

\begin{document}
  \section{Equation numbering}
  \begin{align}
    a = a\\
    b = b\\
    c = c
  \end{align}

  \section{Second section}
  \begin{gather}
    d = d
  \end{gather}
\end{document}

Answer 2

The amsmath package offers you the subequations environment; to get the desired formatting, you can redefine \theequation:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{subequations}
\begin{align}
y & = d \\
y & = cx+d \\
y & = bx^{2}+cx+d \\
y & = ax^{3}+bx^{2}+cx+d
\end{align}
\end{subequations}

\begin{subequations}
\renewcommand{\theequation}{\theparentequation.\arabic{equation}}
\begin{align}
y & = d \\
y & = cx+d \\
y & = bx^{2}+cx+d \\
y & = ax^{3}+bx^{2}+cx+d
\end{align}
\end{subequations}

\end{document}