Definition:Probability Density Function/Naive Definition

Definition
Let $\struct {\Omega, \Sigma, \Pr}$ be a probability space.

Let $X: \Omega \to \R$ be an absolutely continuous random variable on $\struct {\Omega, \Sigma, \Pr}$.

Let $F_X$ be the cumulative distribution function of $X$.

Let $\mathcal S$ be the set of points at which $F_X$ is differentiable.

We define:


 * $\forall x \in \R: \map {f_X} x = \begin {cases}

\map {F_X'} x & : x \in \mathcal S \\ 0 & : x \notin \mathcal S \end {cases}$

where $\map {F_X'} x$ denotes the derivative of $F_X$ at $x$.