Align multiple subequations with each other

Question

Related: Alignment of equals sign in multiple align environments

I have 4 equations that I manipulate. Each equation should be in their own subequation environment (that is, manipulation steps should not be receive a new equation number, but a), b), ... instead.

I'd like to be those 4 manipulations blocks aligned with each other. The related answer above achieves this by planting everything inside one big align environment. However, then I get consecutive numbers as a label (1, 2, 3, 4... and not 1.a 1.b ... 2.a 2.b .... )

Example (not aligned with each other):

\begin{subequations}
\begin{align}
R_1&=(a+b)^2 - 2ab\\
&=a^2+2ab + b^2 - 2ab\\
&=a^2+b^2
\end{align}
\end{subequations}

\begin{subequations}
\begin{align}
R_2&=(a+b)(a-b)\\
&=a^2 + ab - ab + b^2\\
&=a^2 + b^2
\end{align}
\end{subequations}

I tried to nest multiple subequations within an align environment, but that printed equation (verbatim in italics) instead of my actual formula. I also tried to throw everything into one subequations environment and manipulate the equation numbering with addtocounter but that only affected equations thereafter.

Answer 1

Here's a command \stepsubequation that should do what you want. I added also hyperref to show it also works in this case, but is not necessary for the job.

The \stepsubequation can receive as optional argument a label for referring to the following group of subequations, similar to adding \label{foo} after \begin{subequations}.

\documentclass{article}
\usepackage{amsmath}
\usepackage{hyperref}

\makeatletter
\newcommand{\stepsubequation}[1][]{%
  \ifmeasuring@
  \else
    \refstepcounter{parentequation}%
    \protected@xdef\theparentequation{\arabic{parentequation}}%
    \ifdefined\theHparentequation
      \protected@xdef\theHparentequation{\arabic{parentequation}}%
    \fi
    \setcounter{equation}{0}%
    \if\relax\detokenize{#1}\relax\else
      \edef\@currentlabel{\theparentequation}%
      \ltx@label{#1}%
    \fi
  \fi
}
\makeatother

\begin{document}

\begin{subequations}
\begin{align}
R_1&=(a+b)^2 - 2ab \label{a}\\
&=a^2+2ab + b^2 - 2ab \label{b}\\
&=a^2+b^2 \\
\stepsubequation[foo]
R_2&=(a+b)(a-b) \label{c}\\
&=a^2 + ab - ab + b^2 \\
&=a^2 + b^2
\end{align}
\end{subequations}

\begin{equation}
\text{this should have number 3}
\end{equation}

\eqref{a} -- \eqref{b} -- \eqref{c} -- \eqref{foo}

\end{document}

If you do operations such as \counterwithin or \numberwithin to equation, you can do as follows.

\documentclass{book}
\usepackage{amsmath}
\usepackage{xpatch}
\usepackage{hyperref}

\counterwithin{equation}{chapter}

\makeatletter
\AtBeginDocument{%
  \NewCommandCopy{\standardtheequation}{\theequation}%
  \xpatchcmd{\standardtheequation}{equation}{parentequation}{}{}%
}
\newcommand{\stepsubequation}[1][]{%
  \ifmeasuring@\else
    \refstepcounter{parentequation}%
    \protected@xdef\theparentequation{\standardtheequation}%
    \ifdefined\theHparentequation
      \protected@xdef\theHparentequation{\standardtheequation}%
    \fi
    \setcounter{equation}{0}%
    \if\relax\detokenize{#1}\relax\else
      \edef\@currentlabel{\theparentequation}%
      \ltx@label{#1}%
    \fi
  \fi
}
\makeatother

\begin{document}

\chapter{Experiment}

\begin{subequations}
\begin{align}
R_1&=(a+b)^2 - 2ab \label{a}\\
&=a^2+2ab + b^2 - 2ab \label{b}\\
&=a^2+b^2 \\
\stepsubequation[foo]
R_2&=(a+b)(a-b) \label{c}\\
&=a^2 + ab - ab + b^2 \\
&=a^2 + b^2
\end{align}
\end{subequations}

\begin{equation}
\text{this should have number 3}
\end{equation}

\eqref{a} -- \eqref{b} -- \eqref{c} -- \eqref{foo}

\end{document}

Answer 2

Alternatively to @egreg's answer, you could make size of both equations the same

\documentclass{report}
\usepackage{mathtools}
\usepackage{kantlipsum}

\counterwithin{equation}{chapter}


\begin{document}
\chapter{Chapter title}
\kant[1][1]

\begin{subequations}
\begin{align}
    R_1&=(a+b)^2 - 2ab\\
    &=a^2+2ab + b^2 - 2ab\\
    &=a^2+b^2
    \end{align}
\end{subequations}
\begin{subequations}
\begin{align}
    R_2&=
    \mathrlap{(a+b)(a-b)}           % Overwrites empty space
    \phantom{a^2+2ab + b^2 - 2ab}   % allocates empty space
    \\
    &=a^2 + ab - ab + b^2\\
    &=a^2 + b^2
\end{align}
\end{subequations}

\kant[1][2]
\end{document}