ThmDex – An index of mathematical definitions, results, and conjectures.
Borel-Cantelli zero-one law
Formulation 0
Let $P = (\Omega, \mathcal{F}, \mathbb{P})$ be a D1159: Probability space such that
(i) $E_0, E_1, E_2, \ldots \in \mathcal{F}$ are each an D1716: Event in $P$
(ii) $E_0, E_1, E_2, \ldots$ is an D1720: Independent event collection in $P$
Then
(1) \begin{equation} \sum_{n \in \mathbb{N}} \mathbb{P}(E_n) = \infty \quad \implies \quad \mathbb{P} \left( \bigcap_{n = 1}^{\infty} \bigcup_{m = n}^{\infty} E_m \right) = 1 \end{equation}
(2) \begin{equation} \sum_{n \in \mathbb{N}} \mathbb{P}(E_n) < \infty \quad \implies \quad \mathbb{P} \left( \bigcap_{n = 1}^{\infty} \bigcup_{m = n}^{\infty} E_m \right) = 0 \end{equation}
Formulation 1
Let $P = (\Omega, \mathcal{F}, \mathbb{P})$ be a D1159: Probability space such that
(i) $E_0, E_1, E_2, \ldots \in \mathcal{F}$ are each an D1716: Event in $P$
(ii) $E_0, E_1, E_2, \ldots$ is an D1720: Independent event collection in $P$
Then
(1) \begin{equation} \sum_{n \in \mathbb{N}} \mathbb{P}(E_n) = \infty \quad \implies \quad \mathbb{P} \left( \limsup_{n \to \infty} E_n \right) = 1 \end{equation}
(2) \begin{equation} \sum_{n \in \mathbb{N}} \mathbb{P}(E_n) < \infty \quad \implies \quad \mathbb{P} \left( \limsup_{n \to \infty} E_n \right) = 0 \end{equation}
Proofs
Proof 0
Let $P = (\Omega, \mathcal{F}, \mathbb{P})$ be a D1159: Probability space such that
(i) $E_0, E_1, E_2, \ldots \in \mathcal{F}$ are each an D1716: Event in $P$
(ii) $E_0, E_1, E_2, \ldots$ is an D1720: Independent event collection in $P$