 Dynkin system

A Dynkin system contains $\Omega$, is closed under complements, and is closed under countable disjoint unions. Equivalently, it contains differences $B\setminus A$ whenever $A\subseteq B$ are members.

A Dynkin system (also known as a π-system or a Dynkin π-system) is a collection of sets that satisfies certain properties, making it useful in measure theory and probability. Specifically, a collection \( \mathcal{D} \) of subsets of a given set \( X \) is called a Dynkin system if it satisfies the following properties: 1. **Contains the entire set**: \( X \in \mathcal{D} \).