factorial

for a positive integer $\text{[math]}$ , $\text{[math]}$ (read $\text{[math]}$ factorial) is the product of all of the positive integers less than or equal to $\text{[math]}$ . that is,
$\text{[math]}$ furthermore, we define $\text{[math]}$ .
[cite:;taken from @berger_inference_2002 definition 1.2.16]