Reprinting equations

Question

Suppose I have an equation like :

\begin{align} \label{eq:ajk}
a_{jk} = a_{j} - a_{k} %suppose this is a very very long equation with many lines
\end{align}

How can I print the exact same equation again later in my document without actually writing the equation again?

I am looking for something like : \ref{eq:ajk} with the desired output that the equation get's reprinted with the SAME equation number and not just an hyperlink to the equation. I am using amsmath package and it's fine if I need to use any other package.

Is this possible?

Answer 1

To add to this post to get display-style repetitions---

\documentclass{article}
\usepackage{amsmath}

\makeatletter
%%  The basic form (https://tex.stackexchange.com/a/75920/146828)
\newcommand{\repeatable}[2]{%
  \label{#1}\global\@namedef{repeatable@#1}{#2}#2
                           }
%%  In-line style (https://tex.stackexchange.com/a/75920/146828)
\newcommand{\eqrepeat}[1]{%
  \@ifundefined{repeatable@#1}
                 {$nothing$                         (?)}
                 {$\@nameuse{repeatable@#1}$ \eqref{#1}}}
%%  Display-style (slight modification of the above)
\newcommand{\eqrepalign}[1]{%
  \@ifundefined{repeatable@#1}{
              \begin{align} 
                 No\; such\; equation   \tag{?}
              \end{align} 
                              }{%
              \begin{align} 
               \@nameuse{repeatable@#1}  \tag{\ref{#1}}
              \end{align} 
                               }%
                           }
\makeatother


\begin{document}

\noindent
Suppose we have an equation we'd like to repeat---
%
\begin{align}  \repeatable{eq:good}{
   a_{jk} = a_{j} - a_{k}                          }
\end{align}
%
and some other equation, we'd never repeat---
%
\begin{align}
   f(x) = x^2 + 2x + 1
\end{align}
%
We can re-quote the first as: \eqrepeat{eq:good},\\ 
or in display-style as follows---
%
    \eqrepalign{eq:good}
%
but in case of an error it gives: \eqrepeat{eq:bad},\\ 
and displaying a dummy message---
%
    \eqrepalign{eq:bad}
%
%
%
However, the next equation is naturally numbered\ldots
%
\begin{align}
   a^n + b^n = c^n
\end{align}
%

\end{document}

to get an output that looks like--