Subequations, tags and numbering

Question

I have a problem with the following code.

\begin{subequations}
\begin{empheq}{align} 
& a = b \label{E1} \\
& c = d \label{E2}
\end{empheq}
\end{subequations}

\begin{subequations}
\begin{empheq}{align} 
& \alpha = \beta \tag{\ref{E1}$^\prime$} \label{G1} \\
& \gamma = \delta \tag{\ref{E2}$^\prime$} \label{G2}
\end{empheq}
\end{subequations}

\begin{equation}
F(a,c) = 0 \label{E3}
\end{equation}

My problem concerns the equation {E3} since I obtain the sequence of equations tags (1a), (1b), (1a'), (1b') and (3) but i would like to have a tag like (2) on the last equation line. I tried to change the equation counter but I have no results.

I thank you for your help

Answer 1

The subequations environment increments the equation counter every time with a new instance. You can use only one subequations environment. If you want the separation between the two sets add a \jot as shown below:

\documentclass{article}
\usepackage{empheq}
\begin{document}
  \begin{subequations}
\begin{empheq}{align}
& a = b \label{E1} \\
& c = d \label{E2} \\[2\jot]
& \alpha= \beta\tag{\ref{E1}$^\prime$} \label{G1} \\
& \gamma= \delta\tag{\ref{E2}$^\prime$} \label{G2}
\end{empheq}
\end{subequations}

\begin{equation}
F(a,c) = 0 \label{E3}
\end{equation}
\end{document}

If you insist on using two subequations environments, put a \addtocounter{equation}{-1} like:

\addtocounter{equation}{-1}
\begin{equation}
F(a,c) = 0 \label{E3}
\end{equation}

in your code.