# how to understand the error info "Illegal parameter number in definition of \reserved@a. ...t{2+\cdots+\sqrt{2}}}}_{n项} , \cdots 收敛.$}" error info: Illegal parameter number in definition of \reserved@a. ...t{2+\cdots+\sqrt{2}}}}_{n项} , \cdots 收敛.$}


this line's content:

\section{$证明极限:x_{1}=\sqrt{2}, x_{2}=\sqrt{2+\sqrt{2}}, \cdots x_{n}=\underbrace{\sqrt{2+\sqrt{2+\cdots+\sqrt{2}}}}_{n项} , \cdots 收敛.$}


how to understand the error info?

what is "Illegal parameter number" refers to ?

I put it in \section because I want it show in contents.

I think that the Chinese characters is OK in math mode, and math mode in \section is OK too, because I did this many times only this one has error.

• Just out of idle curiosity: Why do you place a long equation inside a \section instruction?
– Mico
Jul 30, 2019 at 20:36
• Are you sure about the Chinese characters in math mode? Jul 30, 2019 at 20:36
• Equation in section seems very uncommon :) Jul 30, 2019 at 21:25

Whatever is the system you're using for Chinese (here I use xeCJK), you need to \protect the fragile command \underbrace.

Anyway, I find it a bad idea to have such a complicated set of formulas in a title. And no, don't use Chinese characters in math mode.

\documentclass{article}
\usepackage[hmargin=1cm]{geometry}
\usepackage{amsmath}
\usepackage{xeCJK}

\begin{document}

\tableofcontents

\section{证明极限: $x_{1}=\sqrt{2}$, $x_{2}=\sqrt{2+\sqrt{2}}$,\ldots\
$x_{n}=\protect\underbrace{\sqrt{2+\sqrt{2+\dots+\sqrt{2}}}}_{n\text{项}}$,\ldots 收敛.}

\end{document}


Something like this?

\documentclass{article}
\usepackage{amsmath,geometry}
\begin{document}
$x_{1}=\sqrt{2}$,
$x_{2}=\sqrt{2+\sqrt{2}}$, \dots,
$x_{n}=\underbrace{\sqrt{2+\sqrt{2+\dots+\sqrt{2}}}}_{\text{$n$times}}$, \dots

{\Large
$x_{1}=\sqrt{2}$,
$x_{2}=\sqrt{2+\sqrt{2}}$, \dots,
$x_{n}=\underbrace{\sqrt{2+\sqrt{2+\dots+\sqrt{2}}}}_{\text{$n$\mdseries times}}$, \dots}
\end{document}

\documentclass{article}
\usepackage{amsmath,geometry}
\begin{document}
$x_{1}=\sqrt{2}$,
$x_{2}=\sqrt{2+\sqrt{2}}$, \dots,
$x_{n}=\underbrace{\sqrt{2+\sqrt{2+\dots+\sqrt{2}}}}_{\text{$n$times}}$, \dots

{\Large % in lieu of '\section*'
$x_{1}=\sqrt{2}$,
$x_{2}=\sqrt{2+\sqrt{2}}$, \dots,
$x_{n}=\underbrace{\sqrt{2+\sqrt{2+\dots+\sqrt{2}}}}_{\text{$n$\mdseries times}}$, \dots}
\end{document}

• I put it in \section because I want it show in contents Jul 30, 2019 at 20:55