ThmDex – An index of mathematical definitions, results, and conjectures.
Result R5317 on D1159: Probability space
Intersections need not preserve equality in probabilities for events
Formulation 0
Let $P = (\{ 0, 1 \}, \mathcal{F}, \mathbb{P})$ be the D5511: Standard single boolean trial probability space.
Then
(1) \begin{equation} \mathbb{P} \{ 0 \} = \frac{1}{2} \end{equation}
(2) \begin{equation} \mathbb{P} \{ 1 \} = \frac{1}{2} \end{equation}
(3) \begin{equation} \mathbb{P}(\{ 0 \} \cap \{ 1 \}) = \mathbb{P}(\emptyset) = 0 \neq \frac{1}{2} = \mathbb{P} \{ 1 \} = \mathbb{P}(\{ 1 \} \cap \{ 1 \}) \end{equation}
Also known as
Counterexample: two events with equal probabilities whose probabilities differ after intersecting with another event