%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-11 on page II-137 in "Equation groups without alignment".
%%
%% 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{gather}
  D(a,r) \equiv \{ z \in \mathbf{C}
         \colon |z - a| < r \}    \notag \\
  \operatorname{seg} (a, r) \equiv
    \{ z \in \mathbf{C} \colon
      \Im z < \Im a, \ |z - a| < r \}    \\
    C (E, \theta, r) \equiv
    \bigcup_{e \in E} c (e, \theta, r)
\end{gather}
\end{document}