how to remove equation label in subequation?

Question

I have the following commands to create 2 lines of equation. The first line contains 4 equations therefore it should be labeled as (1a-d). The second line contains 2 equations therefore it should be labeled as (1f-e). But since subequation has automatic labeling it ruins everything.

{\begin{subequations}\label{eq:1}
\begin{equation}
\refstepcounter{equation}\tag{\theequation a-d}\
\bar{\eta}_1(\frac{\mathcal{L}}{2}) = \bar{\eta}_3(0), \qquad \frac{\partial \bar{\eta}_1}{\partial X}(\frac{\mathcal{L}}{2}) = \frac{\partial \bar{\eta}_3}{\partial X}(0), \qquad \eta(\frac{\mathcal{L}}{2})=0, 
\end{equation}
\begin{equation}
\refstepcounter{equation}\tag{\theequation e-f}\
\qquad \eta(0) = \bar{\eta}_1(0) = \frac{1}{2}.
\bar{\eta}_3(\infty)=0.
\end{equation}
\end{subequations}}

Answer 1

Use \ref:

\documentclass{article}
\usepackage{amsmath}

\newcommand{\pder}[2]{\frac{\partial #1}{\partial #2}}

\begin{document}

Here are six equations grouped in a fancy way
\begin{subequations}\label{double}
\begin{gather}
\bar{\eta}_1\Bigl(\frac{\mathcal{L}}{2}\Bigr) = \bar{\eta}_3(0), \qquad
\pder{\bar{\eta}_1}{X}\Bigl(\frac{\mathcal{L}}{2}\Bigr) = \pder{\bar{\eta}_3}{X}(0), \qquad
\eta\Bigl(\frac{\mathcal{L}}{2}\Bigr)=0,
\tag{\ref{double}a--c}
\\
\eta(0) = \frac{1}{2},\qquad
\bar{\eta}_1(0) = \frac{1}{2}, \qquad
\bar{\eta}_3(\infty)=0.
\tag{\ref{double}d--f}
\end{gather}
\end{subequations}
and here is another, just to check that the numbering is in synch
\begin{equation}
0=0
\end{equation}

\end{document}