! LaTeX 错误：\mathbb 仅允许在数学模式下使用

Question

你好，我遇到了以下代码错误：

$F(X) = \mathbf{1}_{(c,\infty)} := 
\begin{cases}
1 & if\ x \in [c,\infty) \\
0 & if\ x \not \in [c, \infty)
\end{cases} $  

错误说：

! LaTeX Error: \mathbf allowed only in math mode.

 See the LaTeX manual or LaTeX Companion for explanation.
 Type  H <return>  for immediate help.
 ...                                              

 l.282     \$F(X) = \mathbf{1}
                         \_\{(c,\infty)\} :=
? 
! Emergency stop.
...                                              

l.282     \$F(X) = \mathbf{1}
                         \_\{(c,\infty)\} :=
!  ==> Fatal error occurred, no output PDF file produced!   
Transcript written on notebook.log.

希望有人可以帮忙，谢谢！