When using autonum for equation numbering, how to manually add number when not referenced

Question

I use autonum package to number the equations only when they are referenced. However, sometimes I want to refer to a range of equations. For example, I have 4 equations and I type \eqref{eq1}-\eqref{eq4}. But, the 2 equations in between will not be numbered, since they are not referenced. Is there some way to manually number these equations as well?

Here is a minimal example

\documentclass{article}
\usepackage{autonum}
\begin{document}
\begin{equation}
equation 1  \label{e1}
\end{equation}
\begin{equation}
equation 2
\end{equation}
\begin{equation}
equation 3
\end{equation}
\begin{equation}
equation 4  \label{e4}
\end{equation}

I want to refer to \eqref{e1}-\eqref{e4}, but the middle two will not be numbered with autonum.
\end{document}

Answer 1

The following example provides \retainlabel{<label>} which acts like \label{<label>} but also immediately sets a \ref{<label>} inside a box. The \ref is never set, but called and therefore not removed by autonum:

\documentclass{article}

\usepackage{amsmath,autonum}

\newcommand{\retainlabel}[1]{\label{#1}\sbox0{\ref{#1}}}

\begin{document}

\begin{equation}
  f_1(x) = ax^2 + bx + c \label{eqn:one}
\end{equation}
\begin{equation}
  f_2(x) = ax^2 + bx + c \retainlabel{eqn:two}
\end{equation}
\begin{equation}
  f_3(x) = ax^2 + bx + c \retainlabel{eqn:three}
\end{equation}
\begin{equation}
  f_4(x) = ax^2 + bx + c \label{eqn:four}
\end{equation}

See \eqref{eqn:one}--\eqref{eqn:four}.

\end{document}

Answer 2

autonum offers "+" variants of math environments, including equation+ which get numbered regardless of being referenced or not:

\documentclass{article}
\usepackage{autonum}
\begin{document}
\begin{equation}
equation 1  \label{e1}
\end{equation}
\begin{equation+}
equation 2
\end{equation+}
\begin{equation+}
equation 3
\end{equation+}
\begin{equation}
equation 4  \label{e4}
\end{equation}
I want to refer to \eqref{e1}--\eqref{e4}, and now the middle two will also be
numbered with autonum.
\end{document}

Answer 3

\documentclass{article}



\usepackage{hyperref}
\usepackage{cleveref}
\usepackage{autonum}



\begin{document}

\begin{equation}
a\label{a}
\end{equation}
\begin{equation}
b\label{b}
\end{equation}
\begin{equation}
c\label{c}
\end{equation}

Consider \cref{a,b,c}.
\end{document}

I've included hypertext because the ordering of packages is important when you use autonum. Cleveref is one of my favorite packages.