The exponential probability distribution function with parameter $\theta \in (0, \infty)$ is the D4367: Unsigned real function \begin{equation} \mathbb{R} \to [0, 1], \quad x \mapsto \begin{cases} 1 - e^{- \theta x}, \quad & x \in [0, \infty) \\ 0, \quad & x \in (-\infty, 0) \end{cases} \end{equation}