Definition 19.3.1 ($\pi$-System).label Let $X$ be a set and $\mathcal{P}\subset 2^{X}$, then $\mathcal{P}$ is a $\pi$-system if:
- (P1)
$\emptyset \in \mathcal{P}$.
- (P2)
For any $A, B \in \mathcal{P}$, $A \cap B \in \mathcal{P}$.
Definition 19.3.1 ($\pi$-System).label Let $X$ be a set and $\mathcal{P}\subset 2^{X}$, then $\mathcal{P}$ is a $\pi$-system if:
$\emptyset \in \mathcal{P}$.
For any $A, B \in \mathcal{P}$, $A \cap B \in \mathcal{P}$.
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