%%
%% The LaTeX Companion, 3ed
%%
%% Example 12-2-1 on page II-235 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{348.0pt}
%%
%% Examples normally use Times, but here we explicitly
%% want Computer Modern:
\renewcommand\rmdefault{cmr}
%%
%% Also the math axis in the headings is slightly too low,
%% so here is a little command that allows you to change font dimens
%% using NFSS conventions. You are welcome to use it, but be sure you
%% understand the implications when messing with these things :-)
%% No support!
%%
\makeatletter
\def\setfontdimen#1#2#3#4#5#6#7{\begingroup
   \edef\font@name{\csname #2/#3/#4/#5/#6\endcsname}%
   \pickup@font
   \fontdimen#1\font@name=#7\relax
   \endgroup}
\makeatother

\setfontdimen{22}{OMS}{cmsy}{m}{n}{14.4}{4.3pt}

%StartShownPreambleCommands
\usepackage{amsmath,amssymb,bm}
\newcommand\bfB{\mathbf{B}}   \newcommand\bfx{\mathbf{x}}
\bmdefine\bpi{\pi}            \bmdefine\binfty{\infty}
%StopShownPreambleCommands

\begin{document}
\section{The bold equivalence
  $\sum_{j < B} \prod_\lambda : \bm{\sum_{x_j} \prod_\lambda}$}
\begin{gather}
  B_\infty + \pi B_1 \sim  \bfB_{\binfty} \bm{+}\bpi \bfB_{\bm{1}}
    \bm {\sim B_\infty + \pi B_1}                                   \\
  B_\binfty + \bpi B_{\bm{1}} \bm{\in} \bm{\biggl\lbrace}
    (\bfB, \bfx) : \frac {\partial \bfB}{\partial \bfx}
    \bm{\lnapprox} \bm{1} \bm{\biggr\rbrace}
\end{gather}
\end{document}