D-System

A Dynkin system, named after Eugene Dynkin, is a collection of subsets of another universal set Ω {\displaystyle \Omega } satisfying a set of axioms weaker than those of 𝜎-algebra. Dynkin systems are sometimes referred to as 𝜆-systems (Dynkin himself used this term) or d-system. These set families have applications in measure theory and probability. A major application of 𝜆-systems is the π-𝜆 theorem, see below.