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Dynkin's pi-lambda theorem
Statement
Let be a pi-system on . Then . Consequently, if is any lambda-system on with , then .
Facts & Assumptions
Given: A pi-system on , its generated lambda-system , and an arbitrary lambda-system containing .
If is a pi-system on , then is closed under binary intersections (The lambda-system generated by a pi-system is closed under finite intersections).
A lambda-system on closed under binary intersections is a sigma-algebra on (A lambda-system closed under finite intersections is a sigma-algebra).
The family is the smallest lambda-system containing (The generated lambda-system exists and is minimal).
The family is the smallest sigma-algebra containing (Nonempty intersections of sigma-algebras are sigma-algebras, so the generated sigma-algebra exists and is minimal).
Proof
By [L1], [L2], and [L3], is a sigma-algebra containing . Hence [L4] gives .
The sigma-algebra is a lambda-system: it contains , is closed under relative differences because it is closed under complements and intersections, and is closed under increasing countable unions. Since it contains , [L3] gives .
Steps 1.1 and 1.2 prove equality. Minimality in [L3] also gives , so .
Depends on
- The lambda-system generated by a pi-system is closed under finite intersections
- A lambda-system closed under finite intersections is a sigma-algebra
- The generated lambda-system exists and is minimal
- Nonempty intersections of sigma-algebras are sigma-algebras, so the generated sigma-algebra exists and is minimal
Used by
- A C¹ diffeomorphism satisfies the change-of-variables formula for L¹ functions Corollary
- Standard borel spaces have countable generating and measure determining algebras Corollary
- Weak limits are unique Corollary
- Gaussian AR(1) chain Example
- Radial second moment of multidimensional Brownian motion Example
- Successive Brownian exit segments are independent copies Example
- Conditional-independence equivalences and preservation Lemma
- Conditional-independence splice lemma Lemma
- Conditioning a known state and independent noise Lemma
- Finite measures agreeing on a generating pi-system and on the whole space are equal Lemma
- Levy prokhorov distance is a metric Lemma
- Rational conditional distribution functions produce real regular kernels Lemma
- Regular conditional kernels factor through a standard borel conditioning variable Lemma
- Assuming countable and dependent choice, countable products of arbitrary probability spaces Theorem
- Assuming countable choice, Borel probability measures on Polish spaces are inner regular Theorem
- Assuming the Axiom of Choice, Kolmogorov extension for arbitrary families of standard Borel coordinate spaces Theorem
- Borel harmonicity and comparison of harmonic measure Theorem
- Brownian reflection principle Theorem
- Brownian time inversion Theorem
- Brownian-filtration martingale representation Theorem
- Density of elementary predictable processes in predictable L2 Theorem
- Direct integrals of measurable Hilbert fields are Hilbert spaces Theorem
- Disintegration of a joint law on standard borel spaces Theorem
- Existence of regular conditional distributions for standard borel targets Theorem
- Finite-dimensional distributions determine a process law on the cylinder sigma-algebra Theorem
- Finite-dimensional laws of a Markov chain Theorem
- Future-path Markov property Theorem
- Independent pi-systems generate independent sigma-algebras Theorem
- Ionescu-Tulcea construction of a Markov chain Theorem
- Markov property for bounded future path functionals Theorem
- Measurability of integration against a kernel Theorem
- Portmanteau theorem Theorem
- Rapid filters are not Lebesgue measurable Theorem
- Strong Markov property of Brownian motion Theorem
- Uniqueness of Wiener measure Theorem
Dependency tree · two levels
10 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- A. Dembo, Probability Theory lecture notes, Theorem 1.1.38 (standard reference, not scraped)