subequations: Equation numbering does not start with no. 1

Question

The user egreg has posted a nice approach on how to increase equation numbers within subequations.

---

Minimum Working Example (MWE):

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

\newcommand{\advanceparent}{%
  \stepcounter{parentequation}%
  \setcounter{equation}{0}%
  \xdef\theparentequation{\arabic{parentequation}}%
}

\begin{document}
    \begin{subequations}
        \begin{alignat}{3}
            \advanceparent
            && 1 & =1 &\quad & (\text{Example text}) \\
            && 2 & =2 &      & (\text{Example text}) \\
            \advanceparent
            && 3 & =3 &      & (\text{Example text}) \\
            && 4 & =4 &      & (\text{Example text})
        \end{alignat}
    \end{subequations}
\end{document}

---

Screenshot of the result:

---

Description of the issue:

As you can see in the screenshot, even the very first equation begins with no. 2 instead of no. 1. I have tried to repair this behavior, but I don't get the issue fixed.

How to fix the code so numbering will begin with no. 1 and does continue numbering order of the previous document?

Answer 1

The subequations environment allows to use \label just after it starts, in order to refer to the parent number globally. The idea is exploited in the linked answer.

You don't need this, so you have to explicitly start the business.

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

\newcommand{\advanceparent}{%
  \stepcounter{parentequation}%
  \startparent
}
\newcommand{\startparent}{%
  \setcounter{equation}{0}%
  \xdef\theparentequation{\arabic{parentequation}}%
}

\begin{document}

\begin{subequations}
\begin{alignat}{2}
\startparent
1 & =1 &\quad & \text{(Example text)} \\
2 & =2 &      & \text{(Example text)} \\
\advanceparent
3 & =3 &      & \text{(Example text)} \\
4 & =4 &      & \text{(Example text)}
\end{alignat}
\end{subequations}
\end{document}

Answer 2

You can decrease the counter inside of the subequation environment:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

\newcommand{\advanceparent}{%
  \stepcounter{parentequation}%
  \setcounter{equation}{0}%
  \xdef\theparentequation{\arabic{parentequation}}%
}

\begin{document}
    \begin{subequations}
        \addtocounter{parentequation}{-1}%
        \begin{alignat}{3}
            \advanceparent
            && 1 & =1 &\quad & (\text{Example text}) \\
            && 2 & =2 &      & (\text{Example text}) \\
            \advanceparent
            && 3 & =3 &      & (\text{Example text}) \\
            && 4 & =4 &      & (\text{Example text})
        \end{alignat}
    \end{subequations}

\end{document}

If you don't want to do that manually at every set of equations, then you can hook it into the environment with etoolbox. In the example I have used the begin of alignat because the begin of subequations is still too early (the counter is setup after that).

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

\usepackage{etoolbox}
\AtBeginEnvironment{alignat}{\addtocounter{parentequation}{-1}}

\newcommand{\advanceparent}{%
  \stepcounter{parentequation}%
  \setcounter{equation}{0}%
  \xdef\theparentequation{\arabic{parentequation}}%
}

\begin{document}
    \begin{subequations}
        %\addtocounter{parentequation}{-1}% no longer needed
        \begin{alignat}{3}
            \advanceparent
            && 1 & =1 &\quad & (\text{Example text}) \\
            && 2 & =2 &      & (\text{Example text}) \\
            \advanceparent
            && 3 & =3 &      & (\text{Example text}) \\
            && 4 & =4 &      & (\text{Example text})
        \end{alignat}
    \end{subequations}

\end{document}