%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-6-4 on page II-194 in "Operator and function names".
%%
%% 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{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{207.0pt}

%StartShownPreambleCommands
\usepackage{amsmath}
%% Low-level TeX needed here to cancel
%% the old definition of \csc:
\let \csc \relax
\DeclareMathOperator\csc{cosec}
\newcommand\calQ{\mathcal{Q}}
%StopShownPreambleCommands

\begin{document}
\begin{equation}
  \varlimsup_{n\to\infty}
                 \calQ (u_n, u_n - u^{\#})
  \ge \csc (\calQ' (u^{\#}))
\end{equation}
\end{document}