%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-7-17 on page II-204 in "Ghostbusters\texttrademark ".
%%
%% 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{171.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.3\abovedisplayskip}%
    \setlength\belowdisplayskip{.3\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage{mathtools}
%StopShownPreambleCommands

\begin{document}
\Large \[ \sum_{i+j<a^2} x_{ij} \simeq
          \sum_{\mathclap{i+j<a^2}} x_{ij} \approx
          \sum_{\crampedclap{i+j<a^2}} x_{ij}   \]
\end{document}