%%
%% The LaTeX Companion, 3ed
%%
%% Example 12-2-2 on page II-236 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}

%StartShownPreambleCommands
\usepackage{amsmath,amssymb,newtxtext,newtxmath,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}