Cumulative Distribution Function is Right-Continuous
Theorem
Let $\struct {\Omega, \Sigma, \Pr}$ be a probability space.
Let $X$ be a real-valued random variable on $\struct {\Omega, \Sigma, \Pr}$.
Let $F_X$ be the cumulative distribution function of $X$.
Then:
- $F_X$ is right-continuous.
Proof
Let $x \in \R$.
We show that $F_X$ is right-continuous at $x$.
We use Sequential Right-Continuity is Equivalent to Right-Continuity in the Reals: Corollary, and will show that:
- for all monotone sequences $\sequence {x_n}_{n \mathop \in \N}$, with $x_n > x$ for each $n$, that converge to $x$ we have:
- $\map {F_X} {x_n} \to \map {F_X} x$
Let $\sequence {x_n}_{n \mathop \in \N}$ be a monotone sequences with $x_n > x$ for each $n$ that converges to $x$.
Then $\sequence {x_n}_{n \mathop \in \N}$ is a decreasing sequence.
So:
- $\sequence {\hointl {-\infty} {x_n} }_{n \mathop \in \N}$ is a decreasing sequence of sets.
From Limit of Decreasing Sequence of Unbounded Below Closed Intervals:
- $\ds \bigcap_{n \mathop = 1}^\infty \hointl {-\infty} {x_n} = \hointl {-\infty} x$
Let $P_X$ be the probability distribution of $X$.
Note that $P_X$ is a finite measure.
So, from Measure of Limit of Decreasing Sequence of Measurable Sets, we therefore have:
- $\ds \map {P_X} {\hointl {-\infty} x} = \lim_{n \mathop \to \infty} \map {P_X} {\hointl {-\infty} {x_n} }$
So we obtain:
\(\ds \map {F_X} x\) | \(=\) | \(\ds \map \Pr {X \le x}\) | Definition of Cumulative Distribution Function | |||||||||||
\(\ds \) | \(=\) | \(\ds \map {P_X} {\hointl {-\infty} x}\) | Definition of Probability Distribution | |||||||||||
\(\ds \) | \(=\) | \(\ds \lim_{n \mathop \to \infty} \map {P_X} {\hointl {-\infty} {x_n} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \lim_{n \mathop \to \infty} \map \Pr {X \le x_n}\) | Definition of Probability Distribution | |||||||||||
\(\ds \) | \(=\) | \(\ds \lim_{n \mathop \to \infty} \map {F_X} {x_n}\) | Definition of Cumulative Distribution Function |
So, since $\sequence {x_n}_{n \mathop \in \N}$ was arbitrary:
- for all monotone sequences $\sequence {x_n}_{n \mathop \in \N}$, with $x_n > x$ for each $n$, that converge to $x$ we have:
- $\map {F_X} {x_n} \to \map {F_X} x$
So, from Sequential Right-Continuity is Equivalent to Right-Continuity in the Reals: Corollary:
- $F_X$ is right-continuous at $x$.
$\blacksquare$