> [!tldr] > Given a set $\Omega$, $\mathcal{M} \subseteq \mathcal{P}(\Omega)$ is a **λ-system** if > - $\Omega \in \mathcal{M}$, > - If $A,B \in \mathcal{M}, A \subseteq B$, then $B-A \in \mathcal{M}$, > - If $(A_{n}) \subseteq \mathcal{M}$, $A_{1} \subseteq A_{2} \subseteq \dots$, then $\cup_{n}A_{n} \in \mathcal{M}$.