为啥Beamer生成的ppt有红框

defe{{mathrm{e}}}

\author[Qian Lin, Xianming Sun, Chao Zhou] % (optional, use only with lots of authors)
{Qian Lin\inst{1}\and Xianming Sun\inst{2}\and Chao Zhou\inst{3}}
\title{Horizon-unbiased investment with ambiguity}
%\subtitle{}
%\logo{}
\institute[WHU,ZUEL,NUS] % (optional, but mostly needed
{
    \inst{1}
    School of Economics and Management,   Wuhan University, China
    \and
    \inst{2}
    DSchool of Finance, Zhongnan University of Economics and Law, China
    \and
    \inst{3}
    Department of Mathematics, National University of Singapore, Singapore
}
%\date{}
%\subject{}
%\setbeamercovered{transparent}
%\setbeamertemplate{navigation symbols}{}

\AtBeginSection[]
{
    \begin{frame}
        \frametitle{Table of Contents}
        \tableofcontents[currentsection]
    \end{frame}
}

begin{document}

% begin{frame}[plain]
% maketitle{}
% end{frame}

\frame{\titlepage}

\begin{frame}
    \frametitle{}
    \tableofcontents
\end{frame}

再其他的就是常规的写内容的代码了

@u98171 将代码放在代码块中，需要完整，可复现问题，并且直接可以复制使用的代码