Definitions by cases (piecewise functions) not working

[ankur-anand]

$$ n! = \begin{cases}
1 & \text{if n = 0},\\
(n-1)! \ast n & \text{if n > 0}
 \end{cases} $$

is giving content as n!={1if n = 0, (n−1)!∗nif n > 0 in one line