错误是缺少分隔符（.inserted）。 \end{frame} (\documentclass{beamer})

Question

\begin{frame}
\begin{defi}
The class of symbols $\mathnormal{S^m}$ consist the set of
function $\mathnormal{ P(x,\xi)\in C^\infty(R^N\times R^N)}$  such
that for each compact set $\mathnormal {K\subset R^N}$  and 
all multi-indices
 $\alpha , \beta$ satisfying
$$\mathnormal{|D_x^\beta D_\xi^\alpha P(\xi)| \leq C_{\alpha ,\beta} (\big1+|\xi|)^{m-|\alpha|}}.$$
%where $|\alpha|=\alpha_1+\dots+\alpha_n $ , and $\mathnormal{m\in
\end{defi}
Formula (1) assigns to each symbol $\mathnormal{P(x,\xi)\in S^m}$
a pseudodifferential operator $\mathnormal{ P(x,D)}$ of order
$\mathnormal{m}.$ \newline
\textbf{Example}: The function $\mathnormal{p(\xi)=\sqrt{1+|\xi|^2}}$ is a pseudodifferential
 symbol of order 1. Where $\mathnormal{\big1+|\xi|^2}$ is the
 symbol of $1-\triangle$, and ${\triangle=\partial_{x_1}^2+\ldots +\partial_{x_n}^2}$ is Laplace operator. 
\end{frame}

提前感谢你的帮助

Answer 1

问题在于\big您使用的命令（两次）没有使用分隔符作为参数。

当您想要更改分隔符的大小时，可以使用\bigl \bigr但必须指定所需的分隔符。例如：

\bigl( \bigr) \bigl[ \bigr] \bigl\{ \bigr\}

混合起来没问题：

\bigl( \bigr\} \bigl( \bigr] \bigl\{ \bigr]