Command braket error

I was trying to write this formula:

$braket{A} = frac{1}{n} sum_{i=1}^n A_i$

but this error pops out

! 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., `Ihbox'). Otherwise just continue,
and I'll forget about whatever was undefined.