0.1 \(\lambda\) - System
A \(\lambda\)-system on \(\Omega\) is a set \(D\) of subsets of \(\Omega\) satisfying: 1. \(\Omega \in D\); 2. \(A \in D \implies \Omega \setminus A \in D\); and 3. \(A_{1}, A_{2}, \dots \in D\) and pairwise disjoint then \(\bigcup_{n=1}^\infty A_{n}\in D\).