Customizing font style for numbering and referencing equations

Question

I'm typesetting a bunch of proofs and I want to follow a convention that whenever I introduce a premise it is numbered using a bold font (e.g. (1)), but then when I reference it in text it is written using normal font (e.g. (1)). However, I'm having problems with equations. I figured out how to customize style of equation tag to make it bold, but then when I reference that equation using \ref that reference is also in bold. Here's a minimal example:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}
2 + 2 = 4
\refstepcounter{equation}\tag*{\textbf{(\theequation)}}
\label{foo}
\end{equation}

Reference to equation \ref{foo} is now in bold but I want it to be (1).

\end{document}

Above code produces:

How do I make the reference using a normal (non-bold) font?

Answer 1

One could change tag forms with the help of mathtools but this wouldn't help here directly since the current tag form would influence also the reference, which is what you do not want. One might change the tag form only in displayed equations and restore it to the default afterwards, but I'm not as comfortable in hacking into mathtools as I am with amsmath.

\documentclass[twocolumn]{article} % twocolumn for smaller snapshot

\usepackage{amsmath}

\makeatletter
\renewcommand{\eqref}[1]{\textup{(\ignorespaces\ref{#1}\unskip\@@italiccorr)}}
\def\maketag@@@#1{\hbox{\m@th\normalfont\bfseries#1}}
\makeatother

\begin{document}

\begin{align}
2 + 2 &= 4 \label{foo} \\
1 + 3 &= 4 \label{bar} \tag{test 1} \\
2 + 3 &= 5 \label{baz} \tag*{test 2} \\
3 + 3 &= 6 \label{bla} \tag{$*$}
\end{align}
Compare \ref{foo} and \eqref{foo};
\ref{bar} and \eqref{bar};\\
\ref{baz} and \eqref{baz};
\ref{bla} and \eqref{bla}
\end{document}

Note that the asterisk is of course not bold; if you need something like that, you'll have to make it bold yourself.

Answer 2

You are using amsmath, so you can use the command \eqref{foo} instead of \ref{foo}. That will generate the equation number in parentesis non-bolded. Theres also no need for anything in the equation environment besides \label{foo} this way.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}
  2 + 2 = 4
  \label{foo}
\end{equation}

Reference to equation \eqref{foo}.

\end{document}