$$f ( x ) = \left\{ \begin{array} { l l } 4 e ^ { - 4 x } & \text { for } x \geq 0 \\0 & \text { for } x < 0\end{array} \right.$$

True/False

$$f ( x ) = \left\{ \begin{array} { l l } 4 e ^ { - 4 x } & \text { for } x \geq 0 \\0 & \text { for } x < 0\end{array} \right.$$ is a probability density function.