subequations: How to continue numbering while considering scrbook numbering scheme?

Question

The user daleif has posted a nice approach on how to increase equation numbers when using subequations.

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Minimum Working Example (MWE):

\documentclass{scrbook}
\usepackage{amsmath}
\newcommand\StepSubequations{
  \stepcounter{parentequation}
  \gdef\theparentequation{\arabic{parentequation}}
  \setcounter{equation}{0}
}

\begin{document}

First, some normal equation:

\begin{equation}
    content...
\end{equation}

And in following the subequations:

    \begin{subequations}
        \begin{align}
            1   &=  1\\
            2   &=  2\\
            \StepSubequations
            3   &=  3\\% from here a new equation number should begin
            4   &=  4
        \end{align}
      \end{subequations}

      \begin{equation}
        \label{eq:1}
        a=b
      \end{equation}

\end{document}

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Screenshot of the result:

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Description of the issue:

As you can see, scrbook uses a custom numbering scheme like chapternumber.equationnumber. With the counter modification from daleif this does not work unfortunately.

Is there an option available to consider the common scrbook numbering scheme in subequations as well?

In the upper example the subequation numbers should appear like:

0.2a, 0.2b, 0.3a and 0.3b to match the numbering scheme of scrbook.

Answer 1

You can always add \thechapter to the representation:

\documentclass{scrbook}
\usepackage{amsmath}

\newcommand\StepSubequations{
  \stepcounter{parentequation}
  \gdef\theparentequation{\thechapter.\arabic{parentequation}}
  \setcounter{equation}{0}
}

\begin{document}

First, some normal equation:

\begin{equation}
    content...
\end{equation}

And in following the subequations:

    \begin{subequations}
        \begin{align}
            1   &=  1\\
            2   &=  2\\
            \StepSubequations
            3   &=  3\\% from here a new equation number should begin
            4   &=  4
        \end{align}
      \end{subequations}

      \begin{equation}
        \label{eq:1}
        a=b
      \end{equation}

\end{document}

Answer 2

As a comment to Ulrikes answer,

We can simply copy the normal formatting of the equation number and patch it to use parentequation

\documentclass{scrbook}
\usepackage{amsmath}

\usepackage{etoolbox}
\let\sillymacro\theequation % copy definition, then patch, 
 % \ERROR does not exist, so doc will not compile if patch fails
\patchcmd\sillymacro{equation}{parentequation}{\typeout{patched ok}}{\ERROR}

\newcommand\StepSubequations{
  \stepcounter{parentequation}
  % \gdef\theparentequation{\arabic{parentequation}}
  \gdef\theparentequation{\sillymacro}%
  \setcounter{equation}{0}
}

\begin{document}
...