%%
%% The LaTeX Companion, 3ed
%%
%% Example 12-1-8 on page II-228 in "The world of (Latin) math alphabets".
%%
%% 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{99.0pt}

%StartShownPreambleCommands
\usepackage{amsfonts}  \DeclareMathAlphabet\mathbb{U}{px-ds}{m}{n}
%StopShownPreambleCommands

\begin{document}
$ \lbrace n,m \in \mathbb{N} \mid  \mathfrak{N}_{n,m} \rbrace $
\end{document}