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# Dynkin system

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 Title: Dynkin system Author: World Heritage Encyclopedia Language: English Subject: Collection: Publisher: World Heritage Encyclopedia Publication Date:

### Dynkin system

A Dynkin system, named after Eugene Dynkin, is a collection of subsets of another universal set \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.

The primary relevance of λ-systems are their use in applications of the π-λ theorem.

## Contents

• Definitions 1
• Dynkin's π-λ theorem 2
• Notes 3
• References 4

## Definitions

Let Ω be a nonempty set, and let D be a collection of subsets of Ω (i.e., D is a subset of the power set of Ω). Then D is a Dynkin system if

1. Ω ∈ D,
2. if A, BD and AB, then B \ AD,
3. if A1, A2, A3, ... is a sequence of subsets in D and AnAn+1 for all n ≥ 1, then \bigcup_{n=1}^\infty A_n\in D.

Equivalently, D is a Dynkin system if

1. Ω ∈ D,
3. if A1, A2, A3, ... is a sequence of subsets in D such that AiAj = Ø for all ij, then \bigcup_{n=1}^\infty A_n\in D.

The second definition is generally preferred as it usually is easier to check.

An important fact is that a Dynkin system which is also a π-system (i.e., closed under finite intersection) is a σ-algebra. This can be verified by noting that condition 3 and closure under finite intersection implies closure under countable unions.

Given any collection \mathcal{J} of subsets of \Omega, there exists a unique Dynkin system denoted D\{\mathcal J\} which is minimal with respect to containing \mathcal J. That is, if \tilde D is any Dynkin system containing \mathcal J, then D\{\mathcal J\}\subseteq\tilde D. D\{\mathcal J\} is called the Dynkin system generated by \mathcal{J}. Note D\{\emptyset\}=\{\emptyset,\Omega\}. For another example, let \Omega=\{1,2,3,4\} and \mathcal J=\{1\}; then D\{\mathcal J\}=\{\emptyset,\{1\},\{2,3,4\},\Omega\}.

## Dynkin's π-λ theorem

If P is a π-system and D is a Dynkin system with P\subseteq D, then \sigma\{P\}\subseteq D. In other words, the σ-algebra generated by P is contained in D.

One application of Dynkin's π-λ theorem is the uniqueness of a measure that evaluates the length of an interval (known as the Lebesgue measure):

Let (Ω, B, λ) be the unit interval [0,1] with the Lebesgue measure on Borel sets. Let μ be another measure on Ω satisfying μ[(a,b)] = b − a, and let D be the family of sets S such that μ[S] = λ[S]. Let I = { (a,b),[a,b),(a,b],[a,b] : 0 < ab < 1 }, and observe that I is closed under finite intersections, that ID, and that B is the σ-algebra generated by I. It may be shown that D satisfies the above conditions for a Dynkin-system. From Dynkin's π-λ Theorem it follows that D in fact includes all of B, which is equivalent to showing that the Lebesgue measure is unique on B.

Additional applications are in the article on π-systems.