numbering parentequation of subequations

Question

I would like to change the numbering of the parent equation of the subequations.

\begin{subequations}
\label{eq:Parent}
  \begin{align}
  a& \label{eq:1}\\
  b& \label{eq:2}
  \end{align}
\end{subequations}

I cannot use \tag before \begin{align} since it is not in a math environment.

Answer 1

I'd define a new environment, to which you give the desired label as argument.

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newenvironment{varsubequations}[1]
 {%
  \addtocounter{equation}{-1}%
  \begin{subequations}
  \renewcommand{\theparentequation}{#1}%
  \def\@currentlabel{#1}%
 }
 {%
  \end{subequations}\ignorespacesafterend
 }
\makeatother

\begin{document}
An equation
\begin{equation}
1=1
\end{equation}
Some subequations
\begin{varsubequations}{P1}
\label{eq:Parent}
  \begin{align}
  a& \label{eq:1}\\
  b& \label{eq:2}
  \end{align}
\end{varsubequations}
Another equation
\begin{equation}
2=2
\end{equation}
Now the references: \eqref{eq:Parent}, \eqref{eq:1}, \eqref{eq:2}.
\end{document}