%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-26 on page II-145 in "Vertical space in and around displays".
%%
%% 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{183.0pt}

  % Overwrite the value in \normalsize to better visualize the problem.
  % As that is executed at begin of document we need to make the setting there
  % of course that will only last until the next font size change
  \AtBeginDocument{\setlength\abovedisplayshortskip{0pt}
                   \setlength\belowdisplayshortskip{0pt}}

%StartShownPreambleCommands
% The behavior without amsmath
%StopShownPreambleCommands

\begin{document}
Empty line before display:

\[  a \neq b   \]

Enough space now, but do not rely on it!

\begin{equation} a \neq b \end{equation}

Less space after in this case!
\end{document}