%%
%% The LaTeX Companion, 3ed
%%
%% Example 4-1-42 on page I-287 in "thmtools --- Advanced theorem declarations".
%%
%% 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{159.0pt}
% small headings for the example
  \makeatletter
  \renewcommand\section{\@startsection{section}{1}{\z@}%
    {-1.25ex \@plus-1ex \@minus-.2ex}%
    {.5ex}%
    {\normalfont\large\bfseries}}
  \makeatother

  \usepackage{microtype}  % improve the line breaking in short measure


%StartShownPreambleCommands
\usepackage{amsthm,thmtools}
  \declaretheorem[title=Theorem]{thm}
  \declaretheorem[style=definition,title=Definition,
                  sharenumber=thm]{defn}
  \declaretheorem[style=remark,title=Remark]{rem}
%StopShownPreambleCommands

\begin{document}
\listoftheorems[ignoreall,show={thm,defn},
                title=Definitions and Theorems]
\begin{defn}
 A typographical challenge is a problem that cannot be
 solved with the help of \emph{The \LaTeX{} Companion}.
\end{defn}
\begin{thm}[Main]
      There are no typographical challenges. \end{thm}
\begin{rem} The proof is left to the reader. \end{rem}
\end{document}