Automatic "subequations" for amsmath environments?

Question

Is it possible to have amsmath automatically number equations in "subequations" style, when a multiline construct like

\begin{align}
E &= m c^2 \\
c^2 &= a^2 + c^2
\end{align}

is encountered?

Using the subequations environment explicitly is useful, when it is necessary to refer to a group of equations. When the only reason for using it is a style requirement, that successive equations should be numbered as (13a), (13b), etc., it would however be nice to have this automated away.

Additionally, this would be extremely useful in LyX, where using the Subequations module breaks the layout of the live-preview.

Answer 1

Not a good idea, in my opinion. If LyX is not able to cope with subequation, then don't use it or ask its developers to fix it.

Anyway, you can do it:

\documentclass{article}
\usepackage{amsmath}
\usepackage{etoolbox}

\makeatletter
\let\endalignnosub\endalign
\renewenvironment{align*}{\start@align\@ne\st@rredtrue\m@ne}{\endalignnosub}
\renewenvironment{alignat*}{\start@align\z@\st@rredtrue}{\endalignnosub}
\renewenvironment{xalignat*}{\start@align\@ne\st@rredtrue}{\endalignnosub}
\renewenvironment{flalign*}{\start@align\tw@\st@rredtrue\m@ne}{\endalignnosub}
\appto\endalign{\endsubequations}
\preto\align{\subequations}
\preto\alignat{\subequations}
\preto\xalignat{\subequations}
\preto\flalign{\subequations}
\let\endgathernosub\endgather
\renewenvironment{gather*}{\start@gather\st@rredtrue}{\endgathernosub}
\preto\gather{\subequations}
\appto\endgather{\endsubequations}
\makeatother

\begin{document}

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

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

\end{document}

It's necessary to change all environments, because they depend on \endalign, so one cannot just change it.

But don't. Really.