Reference single symbols in equation?

Question

Is there a way to use the equation counter for referencing single symbols inside the equation instead of the whole equation? For example like this

\documentclass[10pt]{scrartcl}

\usepackage{amsmath}
\usepackage[utf8]{inputenc}

\begin{document}

$$\Phi(x)\underset{(1)}{=}\lambda x~,$$
Reference (1) later in the text.

\end{document}

Whole equations and symbol references should use the same counter, so they cannot interfere with one another and the symbol reference should work in $$...$$ as well as align* environment.

My first attempt implementing this looks like this

\documentclass[10pt]{scrartcl}

\usepackage{amsmath}
\usepackage[utf8]{inputenc}

\newcommand{\oplabel}[1]{\refstepcounter{equation}(\theequation\label{#1})}

\begin{document}

$$\Phi(x)\underset{\oplabel{somename}}{=}\lambda x~,$$
Reference \eqref{somename} later in the text.

\end{document}

and it works for $$...$$, but neither for \begin{align}...\nonumber\end{align} nor for \begin{align*}...\end{align*}.

Is there a way to use the equation counter in the align* environment too? (I know the * stands for "no line counting", but I'd like to manually add references where I need them without numbering every line.)

Answer 1

The reason why \label does not work the expected way is that that amsmath package redefines it.

In order to provide a working version, use the original version of \label which is stored by amsmath into \ltx@label. This will work for align, align* and \[...\] as well.

Side note: Don't use $$...$$, that's deprecated syntax, use \[...\] instead.

\documentclass[10pt]{scrartcl}

\usepackage{amsmath}
\usepackage[utf8]{inputenc}

\makeatletter
\newcommand{\oplabel}[1]{\refstepcounter{equation}(\theequation\ltx@label{#1})}
\makeatother
\begin{document}

\[\Phi(x)\underset{\oplabel{somename}}{=}\lambda x,\]

\begin{align}
  \Phi(x)&\underset{\oplabel{someothername}}{=}\lambda x~ \nonumber
\end{align}

\begin{align*}
  \Phi(x)&\underset{\oplabel{yetanothername}}{=}\lambda x~ 
\end{align*}

Reference \eqref{someothername} or \eqref{somename} or \eqref{yetanothername} later in the text.

\end{document}