%%
%% The LaTeX Companion, 3ed
%%
%% Example 12-2-6 on page II-237 in "bm --- Making bold".
%%
%% 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{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{87.0pt}
\setlength\parindent{0pt}
%StartShownPreambleCommands
\usepackage{bm}  \bmdefine\bhat{\hat}
%StopShownPreambleCommands

\begin{document}
  $\hat a \neq \bm{\hat a} \neq \bm\hat a = \bhat a
                           \neq \bm\widehat a $
\end{document}