按要求\geq 后有空格，但是还是报错
\begin{thm}\label{thm1} (Coles et al., 2001)). Assume that Theorem 1 holds with $\xi$ \textless 0 and let the upper endpoint of F be denoted by $b^ *$. Then, \\ P$\bigg(\frac{M_n-a_n}{b_n} \leq z\bigg)$$\rightarrow$W(z), \qquad n$\rightarrow$+$\infty$,\\ where\\ $$W(z)= \begin{cases} exp \biggl\{-\bigg(\frac{b^*-z}{\sigma} \bigg)^\alpha \biggr\}, & if z\text{\textless$b^*$},\\ 1& if z\text{$\geq $$b^*$ },\\此行老是报错 \end{cases}$$ \end{thm}
后来，改了在公式环境中进行，在\geq 前后加入了$$,报错消除
\begin{thm}\label{thm1} (Coles et al., 2001)). Assume that Theorem 1 holds with $\xi$ \textless 0 and let the upper endpoint of F be denoted by $b^ *$. Then, \\ P$\bigg(\frac{M_n-a_n}{b_n} \leq z\bigg)$$\rightarrow$W(z), \qquad n$\rightarrow$+$\infty$,\\ where\\ $$W(z)= \begin{cases} exp \biggl\{-\bigg(\frac{b^*-z}{\sigma} \bigg)^\alpha \biggr\}, & if z\text{\textless$b^*$},\\ 1& if z\text{$\geq $$b^*$ },\\ \end{cases}$$ \end{thm}
其中得到的公式如下
