我试图写出这个公式：
$\braket{A} = \frac{1}{n} \sum_{i=1}^n A_i$
但会出现这个错误
! Undefined control sequence.
<recently read> \braket
l.254 $\braket
{A} = \frac{1}{n} \sum_{i=1}^n A_i$
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.