[Tex/LaTex] How to put \dots in front equation numbers in amsmath

Question

By default the style is

\renewcommand{\theequation}{\thechapter.\arabic{equation}}

for example

            x+y =4         (2.1)

How to put \dots in front equation numbering such as

            x+y =4      ...(2.1)

while in the text reference remain (2.1).

and how to make it for the array by using \begin{eqnarray} \end{eqnarray}

      x = y+ 4         
         -Z+ E         
        + 4gh     ...(2.1)

Answer 1

The »mathtools« package provides the macros \newtagform and \usetagform which allow to do this quite comfortable.

\documentclass[11pt]{article}
\usepackage[T1]{fontenc}

\usepackage{mathtools}  % loads »amsmath«
\newtagform{dots}{\ldots(}{)}

\begin{document}
  \begin{equation}
    (a+b)^2 = a^2+2ab+b^2 \label{eqn:binomi-one}
  \end{equation}
  \begin{equation}
    (a-b)^2 = a^2-2ab+b^2 \label{eqn:binomi-two}
  \end{equation}
  \begin{equation}
    \usetagform{dots}
    (a+b)(a-b) = a^2-b^2 \label{eqn:binomi-three}
  \end{equation}
%
  The Binomi equations~\eqref{eqn:binomi-one},~\eqref{eqn:binomi-two} and~\eqref{eqn:binomi-three}.
\end{document}

Works of course also with the numbering scheme for equations in report and book.

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