mathtools: tagging with words and a number in align

Question

How does one tag an equation with both words and numbers?

\documentclass{article}
\usepackage{mathtools}
\newenvironment{Dequation}
{%                                                                                  
  \def\tagform@##1{%                                                                
    \maketag@@@{\makebox[0pt][r]{(\ignorespaces##1\unskip\@@italiccorr)}}}%         
  \ignorespaces                                                                     
}
{%                                                                                  
  \def\tagform@##1{\maketag@@@{(\ignorespaces##1\unskip\@@italiccorr)}}%            
  \ignorespacesafterend                                                             
}
\makeatother
\begin{document}
\begin{Dequation}
\begin{align*}
3 &= 3\\
4 &= 4\\
6 &= 6\tag{since $x_0 = 0$}\label{six}
\end{align*}
\end{Dequation}
\end{document}

So I want to reference my equation, but I want the reference to be equation (1). Therefore, it would seem strange if the equation wasn't numbered if someone was looking for that reference.

Answer 1

You should not be abusing \tag to set side comments. Just use an alignment to place them:

\documentclass{article}
\usepackage{mathtools}

\begin{document}
\begin{alignat}{2}
3 &= 3&\qquad& \notag\\
4 &= 4&&\text{(since \( p \) is prime)} \notag\\
6 &= 6&&\text{(since \( x_0 = 0 \))}\label{six}
\end{alignat}

By equation~\eqref{six}\dots

\end{document}

Answer 2

Here is one, rather hackish, way to obtain this dual numbering/tagging label:

\documentclass{article}
\usepackage{mathtools}% http://ctan.org/pkg/mathtools
\newenvironment{Dequation}
{%                                                                                  
  \def\tagform@##1{%                                                                
    \maketag@@@{\makebox[0pt][r]{(\ignorespaces##1\unskip\@@italiccorr)}}}%         
  \ignorespaces                                                                     
}
{%                                                                                  
  \def\tagform@##1{\maketag@@@{(\ignorespaces##1\unskip\@@italiccorr)}}%            
  \ignorespacesafterend                                                             
}
\makeatother
\begin{document}

\refstepcounter{equation}\label{six}
\begin{Dequation}
\begin{align*}
3 &= 3\\
4 &= 4\\
6 &= 6\tag{since $x_0 = 0$\upshape{)\,(}\ref{six}}
\end{align*}
\end{Dequation}
See~\eqref{six}.
\end{document}

It's an easy way out, yet definitely viable means, to obtain the output. You manually step the relevant equation counter outside the Dequation environment and label it. Then use it in reference form as part of the \tag. Note that hyperref might not be happy with this approach.

---

A far more common approach to tagging and labelling is by setting the \tag as a descriptor (and thus part of the equation). Here's a way that doesn't affect the horizontal alignment:

\documentclass{article}
\usepackage{mathtools}% http://ctan.org/pkg/mathtools
\newenvironment{Dequation}
{%                                                                                  
  \def\tagform@##1{%                                                                
    \maketag@@@{\makebox[0pt][r]{(\ignorespaces##1\unskip\@@italiccorr)}}}%         
  \ignorespaces                                                                     
}
{%                                                                                  
  \def\tagform@##1{\maketag@@@{(\ignorespaces##1\unskip\@@italiccorr)}}%            
  \ignorespacesafterend                                                             
}
\makeatother
\newcommand{\eqcomment}[1]{\makebox[0pt][l]{\quad\upshape(#1)}}
\begin{document}

\begin{Dequation}
\begin{align}
3 &= 3 \nonumber \\
4 &= 4 \nonumber \\
6 &= 6 \eqcomment{since $x_0 = 0$}\label{six}
\end{align}
\end{Dequation}
See~\eqref{six}.
\end{document}