%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-35 on page II-150 in "Equation numbering and tags".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{207.0pt}

%StartShownPreambleCommands
\usepackage{amsmath}
%StopShownPreambleCommands

\begin{document}
\begin{align}
      A_1 &= N_0 (\lambda ; \Omega')
             - \phi ( \lambda ; \Omega') \\
      A_2 &= \phi (\lambda ; \Omega')
             \, \phi (\lambda ; \Omega)
             \tag*{ALSO (\theequation)}  \\
      A_3 &= \mathcal{N} (\lambda ; \omega)
\end{align}
\end{document}